Ron was staring over their shoulders. They wheeled around.

With a snap, the man in the shadows closed the door behind them.

A mass of filthy, matted hair hung to his elbows. If eyes hadn't been shining out of the deep, dark sockets, he might have been a corpse. The waxy skin was stretched so tightly over the bones of his face, it looked like a skull. His yellow teeth were bared in a grin. It was Sirius Black.

"Expelliarmus!" he croaked, pointing Ron's wand at them.

Hermione and Harry's wands shot out of their hands, high in the air and Black caught them. Then he took a step closer. His eyes were fixed on Harry.

"I thought you'd come and help your friend," he said hoarsely. His voice sounded as thought he had long ago lost the habit of using it. "Your father would have done the same for me. Brave of you, not to run for a teacher. I'm grateful… it will make everything much easier…"

A dark shadow passed over Harry's face and he started forward; Hermione grabbed him and held him back.

"No Harry!" she gasped in a petrified whisper – she was certain that if she were to let him go it would be to his death.

Ron, who had a hold of Harry's other arm, spoke to Black.

"If you want to kill Harry, you'll have to kill us, too!" he said fiercely, though the effort of standing up had drained him of still more colour and he swayed slightly as he spoke.

Hermione could have sworn that she saw something flicker in Black's shadowed eyes.

"Lie down," he said quietly to Ron. "You will damage that leg even more."

"Did you hear me?" Ron said weakly, though he was now leaning heavily on Harry to stay upright. "You'll have to kill all three of us!"

Hermione tried desperately to school her features into something menacing, but felt that she was fighting a losing battle there and gave up.

"There'll only be one murder here tonight," said Black, and that evil grin of his widened.

"Why's that?" Harry spat, trying to wrench himself free of Hermione and Ron. "Didn't care last time, did you? Didn't mind slaughtering all those Muggles to get to Pettigrew… what's the matter, gone soft in Azkaban?"

"Harry," Hermione whimpered, suddenly afraid that her friend had lost his mind. "Be quiet!"

"HE KILLED MY MUM AND DAD!" Harry roared, and with a huge effort he broke free of Hermione and Ron's restraint and lunged forwards, catching Black off balance and propelling them both into the wall.

Hermione heard herself scream; Ron was yelling; the wands in Black's hand sent a jet of sparks into the air, missing Harry's face by inches; Black was twisting madly beneath Harry, trying to break free, but Harry clung on, desperately, his free hand punching every part of Black that he could find.

But Black's free hand had found Harry's throat –

"No," he hissed. "I've waited too long –"

His fingers whitened as they tightened around Harry's neck; Harry choked, his glasses askew.

Making her mind up in an instant, and using tactics she'd learned from her cousin, Hermione kicked Black in the face, hard. At the same time Ron threw himself onto Black's wand hand and knocked the wands to the floor, using his weight to stop Black from grabbing at them. Hermione was employing full playground tactics now and scratched and bit at Black's arm and face until he let her friend go.

Harry pulled himself free of the tangle of bodies and launched himself towards it –

"Argh!"

Crookshanks had joined the fray, apparently on Black's side; he had sunk his claws into Harry's arm, who threw him off – Crookshanks went for Harry's wand but Harry shouted and kicked him out of the way.

"GET OUT OF THE WAY!" he shouted as Ron and Hermione.

They didn't need telling twice. Hermione, gasping for breath, scrambled aside and snatched up her own and Ron's wands; she could taste blood in her mouth and her lip was throbbing where Black had caught her. Ron crawled to the four-poster and collapsed onto it, panting, his white face now tinged with green, both hands clutching his broken leg.

Black was sprawled at the bottom of the wall. His thin chest rose and fell rapidly as he watched Harry walking slowly nearer, his wand pointing straight at Black's heart.

"Going to kill me, Harry?" he whispered.

Harry stopped right above him, his wand still pointing at Black's chest, looking down at him. A livid bruise was rising around Black's left eye and his nose was bleeding.

This is wrong, thought Hermione, with unfamiliar intensity.

"You killed my parents," said Harry, his voice shaking slightly, but his wand hand quite steady.

Black stared up at him out of those sunken eyes.

Harry's going to hurt this man…

"I don't deny it," he said, very quietly. "But if you knew the whole story –"

"The whole story?" Harry repeated, incredulous. "You sold them to Voldemort, that's all I need to know!"

Hermione looked at Harry and saw a boy she didn't know; it frightened her and she edged closer to Ron.

Harry's going to kill this man… and it'll change him… destroy him… even if Black does deserve it…

"You've got to listen to me," Black said, and there was a note of urgency in his voice now. "You'll regret it if you don't… you don't understand…"

…it's the perfect way to murder Harry, to keep him alive, but force him to take his own soul away by killing someone… and then of course, Black would be free of the Dementors…

"I understand a lot better than you think," said Harry, and his voice shook more than ever. "You never heard her, did you? My mum… trying to stop Voldemort killing me… and you did that… you did it…"

She could hear it in his voice, that edge that hadn't been there before, the glint of murder in his eye.

Harry's going to die tonight and there's nothing I can do about it, she thought, with unhappy clarity. Either way, the Harry I know will be dead…

Before anyone could say another word, something ginger streaked past Harry; Crookshanks leapt onto Black's chest and settled himself there, right over Black's heart. Black blinked and looked down at the cat.

"Get off," he murmured, trying to push Crookshanks off him.

But Crookshanks sank his claws into Black's robes and wouldn't shift. He turned his ugly, squashed face to Harry, and looked up at him with those great yellow eyes.

Hermione realised that she was going to lose two of her friends tonight and she sobbed, fingers white around the wands in her hand as she raised them. She had to stop Harry… but to incapacitate him would give Black all the time he needed to seize one of them again. She felt sick –

The seconds lengthened, and still they stood frozen in an appalling tableau: Harry, wand poised at his parents' murderer, Black staring up at him, Crookshanks on his chest; Ron's ragged breathing caught beside her as he saw Hermione, wand raised at her friend, silent, an expression of despair on her face. If Harry made a move…

And then came a new sound –

Muffled footsteps were echoing up through the floor – someone was moving downstairs.

"WE'RE UP HERE!" Hermione screamed suddenly, lowering her wand slightly. "WE'RE UP HERE – SIRIUS BLACK – QUICK!"

Black made a startled movement that almost dislodged Crookshanks, Harry still covering him with his wand – the footsteps were thundering up the stairs now and Harry would be safe… at least from himself.

The door of the room burst open in a shower of red sparks and Harry wheeled around as Professor Lupin came hurtling into the room, his face bloodless, his wand raised and ready. His eyes flickered over Ron, lying on the floor, over Hermione, lowering her wand next to the door, to Harry, standing there with his wand covering Black, and then to Black himself, crumpled and bleeding at Harry's feet.

"Expelliarmus!" Lupin shouted.

Once again, wands flew from their owners' fingers. Lupin caught them all deftly, then moved into the room, staring at Black, who still had Crookshanks lying protectively across his chest.

Hermione was suddenly intensely relieved. Harry hadn't hurt Black; Black hadn't hurt them – well, permanently anyway. He'd be taken away by the Dementors; Professor Lupin would sort everything out –

Then Lupin spoke, in an odd voice, a voice that shook with some suppressed emotion. "Where is he, Sirius?"

Hermione felt her eyes widen, what was going on? Surely Lupin wasn't – but no, Amelia trusted him… didn't she?

Black's face was quite expressionless as he stared at their Professor. For a few seconds, he didn't move at all. Then, very slowly, he raised his empty hand, and pointed straight at Ron. Mystified, Hermione and Harry both glanced at Ron, who looked equally clueless…

Professor Lupin's gone mad, Hermione thought, slowly; she suddenly wished that he hadn't taken their wands.

"But then…" Lupin muttered, staring at Black so intently it seemed he was trying to read his mind, "… why hasn't he shown himself before now?" Lupin's eyes suddenly widened, as though he was seeing something beyond Black, something none of the rest of them could see; Hermione's mind flashed… Ron couldn't be possessed, could he?

Awful realisation appeared to have hit their Professor, "Unless he was the one… unless you switched… without telling me?"

Very slowly, his sunken gaze never leaving Lupin's face, Black nodded.

"Professor Lupin," Harry interrupted loudly, "what's going –"

But he never finished the question, because what he saw next made his voice die in his throat.

Lupin was lowering his wand. Next moment, he had walked to Black's side, seized his hand, pulled him to his feet so that Crookshanks fell to the floor, and embraced Black like a brother.

Hermione felt her insides turn to ice. He'd betrayed her, Harry, Severus, Amelia – them all! He'd betrayed Amelia. Somewhere deep inside her a growl began; blood boiling, she screamed.

"I DON'T BELIEVE IT!"

Lupin let go of Black and turned to her. She was trembling with fury and was pointing at Lupin, wild-eyed. "You – you –"

"Hermione –"

"– you and him!"

"Hermione, calm down –"

"I didn't tell anyone!" Hermione shrieked. "I've been covering up for you –

"Hermione, listen to me, please!" Lupin shouted. "I can explain –"

Harry was in front of her, shaking with venom; his cheeks were flushed and that cold, terrifying light was back in his eyes.

"I trusted you," he shouted at Lupin, his voice wavering out of control, "and all the time you've been his friend!"

"You're wrong," said Lupin. "I haven't been Sirius's friend for twelve years, but I am now… let me explain…"

"NO!" Hermione screamed, "Harry, don't trust him! He's been helping Black get inside the castle, he wants you dead too – he's a werewolf!"

There was a ringing silence. Everyone's eyes were now on Lupin, who looked remarkably calm, though rather pale. Hermione was glaring at Lupin with a look of pure hatred.

"Not at all up to your usual standard, Hermione," he said, quietly. "Only one out of three, I'm afraid. I have not been helping Sirius get into the castle and I certainly don't want Harry dead…" An odd shiver passed over his face. "But I won't deny that I am a werewolf."

Ron made a valiant effort to get up again, but fell back with a whimper of pain. Lupin made towards him, looking concerned, but Ron spat, "Get away from me, werewolf!"

Lupin stopped dead. Then, with an obvious effort, he turned to Hermione and said, "How long have you known?"

Hermione, though still furious, had realised that there were now two lunatic adult wizards against three thirteen-year-olds – she had to keep them talking… Amelia would come.

"Ages," she whispered. "Since I did Professor Snape's essay…"

"Severus will be delighted," said Lupin coolly. "He set that essay hoping someone would realise what my symptoms meant – though he has apologised since… Did you check the lunar chart and realise that I am always ill at the full moon? Or did you realise that the Boggart changed into the moon when it saw me?"

"Both," she said, quietly.

Lupin forced a laugh; it sounded hollow and menacing.

"You're the cleverest witch of your age I've ever met, Hermione – so much like your cousin."

She bridled at the taunt about Amelia. Amelia…

"I'm not," Hermione whispered. "If I'd been a bit cleverer, I'd have told everyone what you are!"

"But they already know," said Lupin. "At least, the staff do."

"Dumbledore hired you when he knew you were a werewolf?" Ron gasped. "Is he mad?"

"Some of the staff thought so," said Lupin. "He had to work very hard to convince certain teachers that I'm trustworthy –"

"AND HE WAS WRONG!" Harry yelled. "YOU'VE BEEN HELPING HIM ALL THE TIME!" He was pointing at Black, who had crossed to the four-poster bed and sunk onto it, his face hidden in one shaking hand. Crookshanks leapt up beside him and stepped onto his lap, purring. Ron edged away from both of them, dragging his leg.

"I have not been helping Sirius," said Lupin. "If you give me a chance, I'll explain. Look –"

Hermione's mind was bridling at the thought that Amelia had known Lupin's secret and not seen fit to tell her. She was horrified that she had trusted him enough to encourage their relationship.

Lupin meanwhile, separated Hermione, Harry and Ron's wands and threw each back to its owner; Hermione caught hers, stunned.

"There," said Lupin, sticking his own wand back into his belt. "You're armed, we're not. Now will you listen?"

Hermione didn't know what to think. Was this a trick?

"If you haven't been helping him," Harry said with a furious glance at Black, "how did you know he was here?"

"The map," said Lupin. "The Marauders' Map. I was in my office, examining it –"

"You know how to work it?" asked Harry, suspiciously.

"Of course I know how to work it," said Lupin, waving his hand impatiently. "I helped to write it. I'm Moony – that was my friends' nickname for me in school."

"You wrote –?"

"The important thing is, I was watching it carefully this evening, because I had an idea that the three of you might try and sneak out of the castle to visit Hagrid before his Hippogriff was executed. And I was right, wasn't I?"

Despite her anger, Hermione blushed; a teacher had caught her breaking the rules… even if he had turned out to be an evil teacher. He had started to pace up and down, looking at them. Little patches of dust rose up at his feet.

"You might have been wearing your father's old Cloak, Harry –"

"How d'you know about the Cloak?"

"The number of times I saw James disappearing under it…" said Lupin, waving an impatient hand again. "The point is, even if you're wearing an Invisibility Cloak you show up on the Marauders' Map. I watched you cross the grounds and enter Hagrid's cottage. Twenty minutes later, you left Hagrid, and set off back towards the castle. But you were now accompanied by somebody else."

"What?" said Harry. "No we weren't!"

"I couldn't believe my eyes," said Lupin, still pacing, and ignoring Harry's interruption. "I thought the map must be malfunctioning. How could he be with you?"

"No one was with us!" said Harry.

But there was one more of us, thought Hermione, suddenly.

"And then I saw another dot, moving fast towards you, labelled Sirius Black… I saw him collide with you. I watched as he pulled the two of you into the Whomping Willow –"

"One of us!" said Ron, angrily.

"No, Ron," said Lupin. "Two of you."

He had stopped his pacing, his eyes moving over Ron almost hungrily.

"Do you think I could have a look at the rat?" he said evenly.

"What?" said Ron. "What's Scabbers got to do with it?"

"Everything," said Lupin. "Could I see him please?"

Ron hesitated, then put a hand inside his robes. Scabbers emerged, thrashing desperately; Ron had to seize his long bald tail to stop him escaping. Crookshanks stood up on Black's lap and made a soft hissing noise.

Lupin moved closer to Ron. He seemed to be holding his breath as he gazed intently at Scabbers.

"What?" Ron said again, holding Scabbers close to him, looking scared. "What's my rat got to do with anything?"

"That's not a rat," croaked Sirius Black suddenly.

"What d'you mean – of course he's a rat –"

What do you think he is, a bloody gerbil? thought Hermione, grumpily.

"No he's not," said Lupin quietly. "He's a wizard."

"An Animagus," said Black, "by the name of Peter Pettigrew."

It took a few seconds for the absurdity of this statement to sink in; then Ron voiced what Harry and Hermione were thinking.

"You're both mental."

"Ridiculous," said Hermione, faintly. If Lupin had gone as mad as he appeared to have he might do anything to them… and yet…

"Peter Pettigrew's dead!" said Harry. "He killed him twelve years ago!"

He pointed at Black, whose face twitched convulsively.

Odd, thought Hermione, for someone supposedly so cruel to flinch at the mention of his crimes…

"I meant to," he growled, his yellow teeth bared, "but little Peter got the better of me… not this time though!"

And Crookshanks was thrown to the floor as Black lunged at Scabbers; Ron yelled in pain as Black's weight fell on his broken leg.

"Sirius, NO!" yelled Lupin, launching himself forwards and dragging Black away from Ron again. "WAIT! You can't do it just like that – they need to understand – we've got to explain –"

"We can explain afterwards!" snarled Black, trying to throw Lupin off, one hand still clawing the air as it tried to reach Scabbers, who was squealing like a piglet, scratching Ron's face and neck as he tried to escape.

"They've – got – a – right – to – know – everything!" Lupin growled, still trying to restrain Black; he doesn't look it, thought Hermione, but he's weirdly strong for a nerdy bloke. "Ron's kept him as a pet! There are parts of it even I don't understand! And Harry – you owe Harry the truth, Sirius!"

Black stopped struggling, though his hollowed eyes were still fixed on Scabbers, who was clamped tightly under Ron's bitten, scratched and bleeding hands.

"All right, then," Black said, without taking his eyes off the rat. "Tell them whatever you like. But make it quick, Remus. I want to commit the murder I was imprisoned for…"

Remus gave Black a dark look.

"You're nutters, both of you," said Ron shakily, looking round at Harry and Hermione for support. "I've had enough of this, I'm off."

He tried to heave himself up on his good leg, but Lupin raised his wand again, pointing it at Scabbers.

"You're going to hear me out, Ron," he said quietly. "Just keep a tight hold on Peter while you listen.

"HE'S NOT PETER, HE'S SCABBERS!" Ron yelled, trying to force the rat back into his front pocket, but Scabbers was fighting to hard; Ron swayed and overbalanced – Hermione watched Harry catch him and push him back down on the bed. Then, ignoring Black, Harry turned to Lupin.

"There were witnesses who saw Pettigrew die," he said. "A whole street full of them…"

"They didn't see what they thought they saw!" said Black savagely, still watching Scabbers struggling in Ron's hands.

"Everyone thought that Sirius killed Peter," said Lupin, nodding. "I believed it myself – until I saw the map tonight. Because the Marauders' Map never lies… Peter's alive. Ron's holding him, Harry."

This explained it then, thought Hermione in horror. Lupin hadn't betrayed their trust; he'd gone mad, desperate to keep his old friend safe once he'd escaped from Azkaban. She shuddered, this was going to destroy Amelia; she couldn't let him do that.

"But Professor Lupin," she said, in a trembling, would-be calm sort of voice, "Scabbers can't be Pettigrew… it just can't be true… you know it can't…"

"Why can't it be true?" Lupin said calmly, as though they were in class and she had simply spotted a problem in an experiment with Grindylows. At this particular moment, Hermione found it rather patronising, but she made an effort to respond in kind.

"Because… because people would know if Peter Pettigrew had been an Animagus. We did Animagi in class with Professor McGonagall. And I looked them up when I did my homework – the Ministry keeps tabs on witches and wizards who can become animals; there's a register showing what animal they become, and their markings and things… and I went and looked Professor McGonagall up on the register, and there have only been seven Animagi this century, and Pettigrew's name wasn't on the list –"

Lupin started to laugh. Hermione frowned; this wasn't a good sign.

"Right again, Hermione!" he said. "But the Ministry never knew that there used to be three unregistered Animagi running around Hogwarts!"

"If you're going to tell them the story, get a move on, Remus," snarled Black, who was still watching Scabbers's every desperate move. "I've waited twelve years, I'm not going to wait much longer."

"All right… but you'll need to help me Pads," said Lupin, "I only know how it began…"

Lupin broke off. There had been a loud creak behind him. The bedroom door had opened of its own accord. All five of them stared at it. Then Lupin strode towards it and looked out into the landing.

"No one there…"

"This place is haunted!" said Ron.

"It's not," said Lupin, still looking at the door in a puzzled way; he sniffed the air. "The Shrieking Shack was never haunted… the screams and howls the villagers used to hear were made by me."

Pushing his greying hair out of his eyes, thought for a moment, then said, "That's where all of this starts – with my becoming a werewolf. None of this could have happened if I hadn't been bitten… and if I hadn't been so foolhardy…"

He looked sober and exhausted. Ron started to interrupt, but Hermione said, "Shh!" She was watching Lupin intently… perhaps he could still be saved.

"I was a very small boy when I received the bite. My parents tried everything, but in those days there was no cure. The Potion that Professor Snape has been making for me is a very recent discovery. It makes me safe, you see. As long as I take it in the week preceding the full moon, I keep my mind when I transform… I am able to curl up in my office, a harmless wolf, and wait for the moon to wane again.

"Before the Wolfsbane Potion was discovered, however, I became a fully fledged monster once a month. It seemed impossible that I would be able to come to Hogwarts. Other parents weren't likely to want their children exposed to me.

"But then Dumbledore became Headmaster, and he was sympathetic. He said that, as long as we took certain precautions, there was no reason I shouldn't come to school…" Lupin sighed, and looked directly at Harry. "I told you, months ago, that the Whomping Willow was planted the year I came to Hogwarts. The truth is that it was planted because I had come to Hogwarts. This house –" Lupin looked miserably around the room, "- the tunnel that leads to it – they were built for my use. Once a month, I was smuggled out of the castle, into this place, to transform. The tree was placed at the tunnel mouth to stop anyone coming across me while I was dangerous.

"My transformations in those days were –were terrible. It is very painful to turn into a werewolf. I was separated from humans to bite, so I bit and scratched myself instead. The villagers heard the noise and the screaming and thought they were hearing particularly violent spirits. Dumbledore encouraged the rumour… even now, when the house has been silent for years, the villagers don't dare approach it…

"But apart from my transformations, I was happier than I had ever been in my life. For the first time ever, I had friends, three great friends. Sirius Black… Peter Pettigrew… and, of course, your father Harry – James Potter.

"Now, my three friends could hardly fail to notice that I disappeared once a month. I made up all sorts of stories. I told them my mother was ill, and that I had to go home to see her… I was terrified they would desert me the moment they found out what I was. But of course, they, like you, Hermione, worked out the truth…

"And they didn't desert me at all. Instead they did something for me that would make my transformations not only bearable, but the best times in my life. They became Animagi."

"My dad, too?" asked Harry, astounded.

"Yes indeed," said Lupin. "It took them the best part of three years to work out how to do it. Your father and Sirius here were the cleverest students in the school, and lucky they were, because the Animagus transformation can go horribly wrong – one reason the Ministry keeps a close watch on those attempting to do it. Peter needed all the help he could get from James and Sirius. Finally, in our fifth year, they managed it. They could each turn into a different animal at will."

"But how did that help you?" said Hermione, puzzled.

"They couldn't keep me company as humans, so they kept me company as animals," said Lupin. "A werewolf is only a danger to people. They sneaked out of the castle every months under James's Invisibility Cloak. They transformed… Peter, as the smallest, could slip between the Willow's attacking branches and touch the knot that freezes it. They would then slip down the tunnel and join me. Under their influence, I became less dangerous. My body was still wolfish, but my mind seemed to become less so while I was with them."

"Hurry up, Remus," snarled Black, who was still watching Scabbers with a horrible sort of hunger in his face.

"I'm getting there, Sirius, I'm getting there… well, highly exciting possibilities were open to us now we could all transform. Soon we were leaving the Shrieking Shack and roaming the school grounds and village by night. Sirius and James transformed into such large animals, they were able to keep a werewolf in check. I doubt whether any Hogwarts students ever found out more about the Hogwarts grounds and Hogsmeade than we did… And that's how we came to write the Marauders' Map, and sign it with our nicknames. Sirius is Padfoot. Peter is Wormtail. James was Prongs."

Hermione heard Harry begin to speak but cut across him.

"That was still really dangerous! Running around in the dark with a werewolf! What if you'd given the others the slip, and bitten somebody?" Despite the situation, she was still a little shocked at Lupin.

"A thought that still haunts me," he said, heavily. "And there were near misses, many of them. We laughed about them afterwards. We were young, thoughtless – carried away with our own cleverness."

"I sometimes felt guilty about betraying Dumbledore's trust, of course… he had admitted me to Hogwarts when no other headmaster would have done so, and he had no idea I was breaking the rules he had set down for my own and others' safety. He never knew I had led three fellow students into becoming Animagi illegally. But I always managed to forget my guilty feelings every time we sat down to plan our next month's adventure. And I haven't changed…"

Lupin's face had hardened, and there was self-disgust in his voice. "All this year, I have been battling with myself, wondering whether I should tell Dumbledore that Sirius was an Animagus. But I didn't do it. Why? Because I was too cowardly. It would have meant admitting that I'd betrayed his trust while I was at school, admitting that I'd led others along with me… and Dumbledore's trust has meant everything to me. He let me into Hogwarts as a boy, and he gave me a job, when I have been shunned all my adult life, unable to find paid work because of what I am. And so I convinced myself that Sirius was getting into the school using dark arts he learned from Voldemort, that being an Animagus had nothing to do with it… so, in a way, Snape's been right about me all along."

"Snape?" said Black sharply, taking his eyes off Scabbers for the first time in minutes and looking up at Lupin. "What's Snape got to do with it?"

"He's here, Sirius," said Lupin heavily. "He's teaching here as well." He looked up at Hermione, Harry and Ron.

"Professor Snape was at school with us. He fought very hard against my appointment here… he told Dumbledore that I was not to be trusted. Though I had hoped… You see, Sirius here played a trick on him which nearly killed him, a trick which involved me..."

Black made a derisive noise.

"It served him right," he sneered. "Sneaking around, trying to find out what we were up to…"

Hermione was struck by how much Snape sounded like her and the others.

"Severus was very interested in where I went every month," Lupin told them. "We were in the same year, you know, and we – er – didn't like each other very much. He especially disliked James's talent on the Quidditch pitch… anyway, Snape had seen me crossing the grounds with Madam Pomfrey one evening as she led me towards the Whomping Willow to transform. Sirius thought it would be – er – amusing, to tell Snape all he had to do was prod the knot on the tree-trunk with a long stick, and he'd be able to get in after me. Well, of course Snape tried it – if he'd got as far as this house, he'd have met a fully grown werewolf – but your father, who'd heard what Sirius had done, went after Snape and pulled him back, at great risk to his life… Snape glimpsed me, though, at the end of the tunnel. He was forbidden to tell anybody by Dumbledore, but from that time on he knew what I was…"

"So that's why Snape doesn't like you," said Harry slowly, "because he thought you were in on the joke?"

"That's right," sneered a cold voice from the wall behind Lupin.

Severus Snape was pulling off the Invisibility Cloak, his wand pointing directly at Lupin.