The month rolled past. Their year group at Hogwarts had settled into a quieter routine than previous years; OWLs were on their way, and even the Slytherin and Gryffindor gangs were sucked into exam prep.

Sev, of course, had little need for any kind of studying. He bided his time, waiting for the perfect moment to strike.

It came. On the day of the full moon, he cornered Sirius again in the early morning.

"I've figured it out," he said with a tone of malicious triumph. "The passage, under the tree - it only opens on a full moon, doesn't it? It's your secret route into Hogsmeade."

Sirius scowled at him. "I don't know what you're talking about." But Sev could detect the faintest gleam of triumph in his eye; a little light that said I know something you don't know.

Under the circumstances, Sev found that idea more than a little ironic.

The groundwork for his little trap had all been meticulously set. Now all that remained was to extend the bait... and see if Sirius snapped it up.

Sev leaned forward threateningly. "I know about your little passage, Black. Lie to my face all you like; it won't make a knut's worth of difference if I report you, and you know it." He gave a nasty grin that was modelled on one of Malfoy's finest. "Tell me how to get in, Black. Tell me how the passage works, and maybe I won't bother bringing a premature end to your worthless school career." He smiled thinly. "I somehow doubt you'll be here for much longer in any case."

That, of course, was a long way from being true. Sirius might be wild to the point of dangerousness, but he was also quick-witted and a surprising natural at passing exams.

Also, apparently, he wasn't too bad at acting, for he managed to pull off a picture of sullen resentment quite well. Of course, that was close to his natural state half the time. Sev blamed it on excess testosterone.

"There's a knot," he said gruffly. "On the trunk. You poke it with a stick."

"Thank you," said Snape, obnoxiously.

Sirius scowled darkly. "You try and stick me with this, Snape, they'll be pulling bits of you out of the lake for the next decade."

"Stop it, you're scaring me," said Sev completely flatly as he walked away. Sirius' shoulders tensed, but he didn't turn back. No doubt because he was hiding a smile of triumph at a trap well sprung.

Sev, on the other hand, was far too practised at this to let any such sign show on the surface.

The year was on the first edge of summer, and it was late before it began to get dark. Sev lurked silently in the grounds, a safe distance from the Whomping Willow. He had cast an enchantment that made him unnoticeable to anyone who wasn't specifically looking, but he doubted it was needed. The Gryffindor boys were entirely too self-centred, anyway.

James had an invisibility cloak - or rather, Lily did, though Sev doubted she would ever refuse to loan it to him - but they weren't using it when they finally appeared. They were all sixteen or nearly there by now; Pete Pettigrew might be small, but Sirius was built like an athlete and the other two were tall. There was no way all four of them could fit under that cloak, Sev knew without a doubt; after all, it had once been in his possession.

As it happened, though, the boys were making no attempt at being sneaky. They were bombing it across the grass as if there were a swarm of dragons on their tail, Remus in the lead.

"Idiot!" he heard Sirius snap, his voice carrying clearly through the still dusk air.

"I said I'm sorry." That was James.

"Back there necking with your girlfriend, man, the moon's nearly up!"

"I was not," he objected.

"Do I care?" Remus's voice was strained almost beyond all recognition. "Open it, open it, open it, bloody hell-"

"There!" A little figure that had to be Peter ducked in under the flailing branches and they abruptly came to a halt. Remus dived right over him and into the secret passage. The others tumbled in after him; laughing, Sev was sure, at their narrow escape.

They were used to playing with fire, although never on the grand scale of peril that he was accustomed to. Whatever trouble they got into, they wriggled their way out of it; they'd never truly had their fingers burned.

Tonight, he intended to give them a little taste of what that was like.

The boys clearly had some way of dealing with Remus when he was under the influence of the moon. Sev was curious, as he always was about anything he didn't know, but he wasn't about to compromise his plan by busting in while the other three were still down there. It would bring a much better confrontation, but for that very reason it would be too suspicious. He was the cruel, devious boy who spied on them and threw curses; they would expect him to sneak, not confront.

Well, he would sneak. He would wait for them to emerge from under the tree, and then go down himself. And then he would 'narrowly escape' what lay beneath - and scream blue murder about it.

It wasn't long before three boyish figures emerged where four had gone in. The Gryffindors might feel the need to show their solidarity with Remus, but they couldn't stay down there all night; they had to sleep sometime.

Sev watched them go, and immediately left his hiding place and headed for the Whomping Willow. He had a hunch Sirius might stick around to watch for him; best to make his move before the other boy got bored and wandered off. It would definitely be handy to have someone to shout at when he emerged...

The limbs of the Whomping Willow resumed their usual flailing. Sev had spent some time watching them, a few years back; so far as he could tell, there was no pattern to the motion at all. If you didn't know the tree's secret, there was no way you could get past them.

Sev spotted the knot Sirius had described almost instantly. It was low down on the trunk, close to the ground - well below eye-level. Smart. Sev suspected Dumbledore's hand in this; it had been the headmaster who had wanted the Whomping Willow planted in the first place. He was fairly sure it had been with the sole intent of providing a safe place for Lupin to stay during the full moon.

Sev drew his wand, and reversed it in his hand to reach out and poke the knot - only an idiot poked things with the business end of a wand. The tree immediately froze.

The hole the boys had disappeared into, he could see from this close, was masquerading as a simple gap between the roots. It was narrow and low, and if anybody had seen it at all they would have thought it an animal burrow. But who would be stupid enough to come so close to the highly dangerous tree?

Sev got down on his belly and squirmed through the hole. It was an easy enough fit for him, although the broad-shouldered Sirius probably had trouble. He slide down a slope inside to end up on his knees in a very low tunnel.

"Lumos." He lit the wand with a low whisper, and looked about. There was little to see; the tunnel stretched out in front of him, a good foot too low for him to walk upright. From the general direction, it was clearly headed for Hogsmeade.

Another piece of the puzzle suddenly slotted into place. The Shrieking Shack. Lately, a legend had sprung up about some howling beast that haunted an abandoned shack in Hogsmeade. Sev was willing to bet that the howling was only heard on nights when the moon was full...

It was quicker to scuttle on his hands and knees than try to move at a stoop. And he would need to be quick; he had a number of tactics in mind for facing the creature he would meet at the other end, but he wasn't sure how well they were going work. Any books he could find were notoriously stingy with information on the subject; generally speaking, their defences were of the "kill it before it kills you" variety.

It would be considerably safer to lurk in the tunnels and then come out pretending to have encountered Lupin, but everything in Snape's calculating nature rebelled against it. He doubted very much Sirius Black was sitting up there timing his journey to make sure he took long enough, but Sev felt obliged to do this properly anyway. He hadn't got as far as he had by being half-hearted with his deceptions.

Besides which, he wasn't sure whether or not Lupin would remember the events of a full moon night. All the books said no, but whilst Sev worshipped books, he trusted them little more than he did people. The key to surviving was to take nothing on face value.

The tunnel went on for a long time; he was mentally gauging it against the length of his own secret passage into Hogsmeade. Soon, surely...? The tunnel suddenly began to slope upwards.

Warily, now, Sev drew his wand and slowed to a creep.

There was a change in the air of the tunnel; the stir of a minor breeze. He was right at the end of the passageway. He moved towards the gap, caught the slightest glint of light in a dark, beady eye surrounded by coarse fur, and then...

Everything exploded into motion. The werewolf ahead of him let out a sound that was more like a scream than a howl, and scrabbled madly at the tunnel entrance as if trying to remember how to pass through it. At the same time, somebody grabbed him by the back of the collar and yanked him backwards.

"Snape! Run!"

It was James Potter.

The two boys scrambled back through the tunnel at high speed. Sev could hear and feel hot, heavy breath directly behind him, but the werewolf seemed to be so huge it was having trouble squeezing itself along the tunnel. James had him by the shoulder, half-pushing, half-dragging him along.

The tunnel was barely wide enough for the two of them to run abreast, especially stooped as they were. He still had his wand in his hand, but if he stopped to cast a spell the beast would be upon them.

There was no time to say anything, and neither he nor James were particularly inclined to stop and chat. They pounded through the tunnel, hardly able to breath in the clouds of dust kicked up by their feet, and once Sev heard James curse loudly as he cracked his head on the ceiling.

The end of the tunnel seemed to come upon them unexpectedly, and James threw himself at it, squirming up through the opening like some kind of burrowing animal. Sev wondered how many times he and the others had done this mad dash when their attempts at calming the werewolf had failed or backfired.

Then James's hands were thrust back through the hole, and he yanked Sev up out of it. His back scraped all along the bottom of the tree roots, and as James pulled him away from the tree one of the flailing branches cracked against his shoulder.

They both collapsed on the grass. James was breathing raggedly, and it took a moment for him to push himself up onto his hands and turn to look at Snape. "Bloody hell, man, are you okay?"

This, right here, was the defining moment. The natural, human instinct would be to check his injuries, mutter some quick meaningless phrase like "Yeah, I think so." It was the automatic response to going through such a blur of chaotic action.

With barely a hesitation, Sev thrust aside all such instincts and drew himself up, fire in his eyes. "You set me up!" he spat.

"I- I- what?" Confusion turned to indignation. "I saved you, man! You could have been killed!"

"Whose fault was that?" he demanded at the top of his voice. "You think this was funny?"

"I came in after you!" James was shouting, too, now, having forgotten that it was night and neither of them had permission to be out.

"Oh, so that makes it okay? Never mind that I could have been torn limb from limb by that creature in there-"

"Hey-!"

"He's a monster, man! Your friend is a monster!" Sev wasn't nearly as stupidly ignorant as to believe all the rubbish that was spouted about werewolves. He knew Remus Lupin well enough, saw that in spite of - probably because of - his deep, dark secret, Remus was an extremely gentle-natured, pacifistic boy. However, a little bit of irrational prejudice was a handy thing to throw into the mix right now. The last thing he needed was James calming down enough to keep this civilised.

Predictably, James exploded. He'd risked his life rescuing Snape from the jaws of death - he wasn't about to take this kind of abuse in return. "Sort. Your head out. Man. It's your own damn fault! If you hadn't been poking your nose in-"

"My fault? You let that, that monster on school grounds and it's my fault when it tries to kill me?"

"If you hadn't been following us around, spying on us-"

"Oh, yeah? Was I down there by accident? Your friend Black told me how to get down there. Was that me poking my nose in? No, that's premeditated murder, that is!"

"Oh, are you dead, are you? No, you're not!" He and James were face to face now, the Gryffidor boy's face red with fury. "Why not? Because I went down and saved you!"

"Oh, yeah, you're a real hero-"

"Hey! Hey! What the in the seven fires of hell is going on here?" At last, their rapidly-escalating shouting match had done what Snape had intended it to - got them some attention. As luck would have it, it was Professor Malachite.

He came looming out of the darkness from the direction of the Forbidden Forest, with thunder in his face. "Potter!" he snapped angrily when he saw James. He came to a dead halt when he realised who the other shouter was. "Severus? What in Merlin's name is going on here?"

"He tried to kill me!" Sev snapped, careful to sound merely furious, not hysterical - Malachite knew him too well to accept that.

"I saved his life!" James shouted right back. Malachite stood there for a long moment, looking from one boy to the other. Finally he snapped his fingers in a moment of decision.

"Both of you, come with me. We're going to the headmaster."