Harry and Hermione spent a very pleasant hour with Molly and Arthur Weasley, catching up on the family news, answering a dozen new questions about Muggle habits, and eating scones and cakes and biscuits until they were in danger of bursting at the seams. It was lovely, as it had always been to be in the Burrow, and it was odd, as it had been to be in the Burrow since the end of the Wizarding War: to be safely wrapped in the Weasley's slightly manic domesticity at the same time as knowing just how fragile that sanctuary really was. Harry found himself looking at Molly and Arthur with an odd sense of seeing them through two different sets of eyes. One moment, they were Ron and Ginny's ever-protective parents, the adults who could swoop to the rescue when needed; the next, they were a powerful witch and wizard whose strengths and skills very nearly approached his own.

His own students were the age Ginny had been when she'd been taken to the Chamber of Secrets; the age he had been when he'd fought the Basilisk; the age Ron had been when his rat had turned out to be a Death Eater and he'd come very close to being bitten by a werewolf.

Merlin's beard, we were young.

For the first time, he could appreciate just how difficult it must have been for Molly and Arthur — for all the adults who cared about us — to maintain that air of reassurance. All of them fought in the first war against Voldemort. They were my parents' friends; they were Alice and Frank Longbottom's friends.

They had seven children to worry and fear for, and they still took Hermione and me to their hearts as if we were their own.

Harry tried to imagine how he'd feel if one of his students — young people he'd known for only a few months, who had parents and families of their own to worry about them and protect them — had been lured into an ambush by Death Eaters the way he and his friends had been. His blood ran cold at the thought.

He might have greater magical talent than either Molly and Arthur Weasley, but in some ways, they were both stronger than Harry could even imagine.

When it came time for him and Hermione to leave, Harry hugged Arthur and then Molly fiercely.

"Harry, is something the matter?" Molly asked.

He pulled back a little to look her in the face, arms still around her. "No." He smiled, to let her know he was telling the truth. "Just — thank you. For everything."

"Oh, Harry." She pulled him close again. "You know you never need to thank us."

"Thanks anyway."

He couldn't find any better words. All he could do was hope they understood.

He and Hermione Apparated back to the Hogwarts gates, and started along the path to the castle, both lost in their own thoughts.

It was Hermione who broke the silence. "Do you think it was a mistake?"

"Did we?" Harry said, startled.

"We could have stayed," Hermione said. "I mean, it felt rather like we were intruding, but we could have stayed."

"For dinner?"

"Well, not, not for dinner, that would have been a bit much, but, you know, for a while."

Harry frowned. "What on earth are you talking about?"

"Leaving Professor — leaving him with Patience." She matched his frown. "Why, what are you talking about?"

Harry shook his head. "Doesn't matter. No, I don't think it was a mistake. I mean, what's the worst that could happen?"

"He could have been wrong about Patience. She could really be out to get him."

"Good luck taking on that particular wizard face-to-face."

Hermione's face was pinched with worry. "What if she does something like poison him?"

"Hermione." Harry put his arm around her shoulders. "You're an expert in Potions, if not to his standards. How many poisons are there that you wouldn't notice in a cup of tea or a biscuit?'

"Four," she said promptly. "No, five."

"Any of those the sort of thing someone would have hanging around the house on the off-chance a deadly enemy would come to call?"

"No," she conceded.

"He's well able to take care of himself. And besides, the whole reason for today was that he wasn't likely to be wrong, wasn't it?" Harry gave Hermione's shoulders a squeeze. "Don't worry so much."

She bit her lip. "You're going to tell Kingsley now, aren't you?"

"Not this second," Harry said.

Hermione twisted a little to look at him, her pace slowing and then stopping. "You've changed your mind?"

He shook his head. "The Ministry needs to know. But Professor S— someone was right. Dumbledore didn't go to the Ministry curse-breakers, did he, or to St Mungo's. He could have asked Mad-eye for help, the greatest Auror of all time, someone he could have trusted to never breathe a word about it. He didn't." With another squeeze of her shoulders, Harry drew Hermione with him along the path. "And if I have to pick someone to break a curse on my best friend, I'm going to go with the man recommended by Albus Dumbledore."

Hermione rubbed her forearm. "So you'll wait until you've finished using him before you turn him in?"

"You are making him one of your causes," Harry said. She frowned at him, and he smiled to take the sting from his words. "He agrees with me, you know. That finding out which Auror is flirting with Dark magic is important. Why else would he change his mind about coming today? I gave him twenty-four hours to give him the chance to disappear, and to be honest, I thought he would."

"Can't you tell Kingsley that he has disappeared?" Hermione asked.

"I suppose I could," Harry said. "I really don't want to lie to him, though."

"Maybe he just won't come back," Hermione said, biting her lip.

So long as she continues to take the potion, the matter is … not urgent, Snape had said. "He'll come back." He has to, doesn't he? To keep brewing that potion for Hermione, if nothing else.

It occurred to him that finding out who'd cursed him and protecting the integrity of Azkaban might not have been the entire reason Severus Snape had agreed to accompany them to Patience Monkshod's house today.

"He'll come back," he said with certainty.

And in fact, when he got back to the rooms he shared with Ron and checked the Marauder's Map, a slowly-moving set of footprints showed him that Snape was already in his own quarters.

Harry raked his fingers through his hair, watching the foot-steps pace from one side of the room to the other, then back, then back again. Snape, standing with his back turned, thin shoulders braced as if for a beating, his voice like a dead man's. "Her sister never did."

Disturbing Snape this evening seemed like a very bad idea.

I owe it to him to tell him he's safe from discovery — from further discovery — at least. He could send a Patronus, but it didn't seem likely that Snape would react well to the sight of James Potter's stage Patronus, even at the best of times.

And is it for his sake that I want to leave him alone, after today?

Or for mine?

He could be grown-up enough to make his way down to the dungeons and knock, once again, on Snape's door, but he couldn't quite make himself to be grown-up enough to be happy about it.

The door opened, which meant he was, if not welcomed, at least tolerated.

Then Harry made his way down the short corridor and through the second door and found Snape glaring at him with a cold malevolence that forced him to revise his reasoning. "Thank you for letting me in, sir," he said as calmly as he could, given that the other man looked as if he wanted nothing more in the world than to murder him. As much as he found it hard to believe Snape would actually harm him, Harry was glad that Snape had stopped his pacing behind his black leather armchair and its squat solidity was between them.

"If I hadn't, you would have attracted attention," Snape all but spat at him.

I'm not that much of an idiot. Harry bit back the words unsaid. "I wanted to tell you that I'm not going to tell Kingsley about you, not yet."

"And what do I have to thank for this small mercy? Is it too much to hope that you've finally learnt to consider the effects of your actions on those other than yourself and your friends?"

So you'll wait until you've finished using him before you turn him in? Hermione's voice came back to him, and Harry winced.

Snape caught it. He gripped the back of the chair until his fingers turned white. "How glad I am that I am still of some use to the Chosen One. Does it occur to you that you have just given me a reason to delay assisting Hermione Granger for as long as I possibly can?"

"It didn't occur to me, no."

"No," Snape sneered. "It wouldn't, would it? Not to the boy noble enough to sacrifice his life for his friends."

Harry raised his eyebrows, his pulse ticking up with anger. "I didn't exactly have much of a choice, you know. It's not like I made a career decision somewhere along the line. 'Shall I go into Magizoology or opt for Dying To Bring An End To Tom Riddle? Hmm, that's a tough one … perhaps I should toss a coin'."

"As if you would have chosen the glory of shovelling Graphorn dung." There was so much acid dripping from Snape's tone that Harry was half-surprised the words didn't burn holes in the ornate rug on their way to him.

He couldn't match it, but he did put as much sarcasm as he could manage in his voice. "As if you would have chosen to run like Karkaroff."

"His flight instigated his ultimate demise, Potter. Please do not embarrass yourself by attributing motives to me beyond self-preservation."

"I know that's —" Harry stopped, and sighed. He was suddenly very tired. "Do we have to keep doing this? I mean, I get it. You don't like me. Of all the Aurors in all the world, I'm the last one you want to have to deal with."

"Congratulations on your perceptiveness," Snape said, soft and venomous. "Would you like a round of applause? Perhaps an engraved trophy?"

Harry looked straight at him, refusing to be intimidated by the loathing and anger so clear in Snape's set face and glittering eyes. "But we're stuck with each other at the moment. So here's the plan. I'm going to need Kingsley Shacklebolt's approval to get Bellatrix Lestrange's knife out of the Ministry vault. If you can think of a good reason I can give him, that'd be a help. In the meantime, if you're certain Patience Monkshod isn't involved —"

"If you're going to doubt my conclusions, Potter, I wonder why you even compelled me to participate in today's fruitless enterprise."

"—then we need to find another way to identify the Auror involved, other than making a Ministry case of it," Harry went on as if Snape hadn't interrupted him, although it took just about all his self-control not to snap back at the miserable git.

Snape's lip curled in a sneer. "Lost your faith in the infallible Ministry for Magic, now? My, it's quite the season for you when it comes to losing your illusions, isn't it, Potter? First your father, now your father substitute."

Harry had always liked and respected Kingsley Shacklebolt, but the idea of him as a substitute father was so absurd that he felt his anger leak away, the blow Snape had intended as a knife to the ribs going so wide that all it did was puncture the illusion that Snape's palpable rage had been about Harry in the first place.

"Yes, it was terrible to realise that he was only interested in my abilities, my experience, and my work ethic," he said equably, and had the satisfaction of seeing Snape's eyes widen in a surprise he couldn't quite conceal. "My secret hopes of being his illegitimate love-child were quite dashed."

For a moment they stared at each other, and then Harry saw the faintest twitch at the corner of Snape's mouth. "Then you must have been denser even than your performance in Potions indicated."

"Haven't you realised yet that Hermione carried me through most of your classes?"

"You and Longbottom both?"

"At least Neville paid attention."

"I shudder to think what his potions would have been like if he hadn't, then," Snape said dryly. Harry noted that his grip on the chair had eased, the tendons on the back of his hands no longer standing out like wires beneath his skin.

"What, two decades teaching hasn't given you any worse examples?"

"None that you're old enough to hear," Snape retorted. The twitch at the corner of his mouth had definitely become a half-smile, and Harry allowed himself to breathe a sigh of relief.

He pulled out his wand and Transfigured one of the books lying by the hearth into an armchair matching Snape's own.

"Flashy," Snape said sourly.

"Surely only what you expect from me?" Harry countered. He sat down, and, after a moment, Snape took his own chair. "Professor. We need to work together on this, at least for now. Hermione told me that you need the knife that made her scar to help lift whatever curse Bellatrix used — I can get that knife, and you can't. I need to identify which Auror at Azkaban is meddling with Dark magic, and without involving the Ministry, I can't do it without your cooperation." Harry paused. I hope I'm right about this. "And we both want each of us to succeed."

Snape crossed his legs in one economical movement and laced his fingers together. "I concede your logic."

Wonders will never cease. "I think I can persuade Kingsley to let me have the knife for a bit, if I tell him what it's for."

"And do you think Granger will forgive you for that?"

"If I'm right, once the curse is broken, yes," Harry said steadily. "If I'm wrong, the curse will still be broken."

Snape inclined his head slightly. "And in return? What is your quid quo pro, Potter?"

Harry frowned. "I don't think I know that spell, sir."

Both Snape's eyebrows went up, as eloquent as if he'd rolled his eyes. "That's because it isn't a spell. It means 'something for something'. An exchange, of equal value."

"That sounds a bit like to me like it ought to be a spell," Harry said. "Well, my pro for your quid is — Aurors are taught, part of our training, to recognise spells cast by the same person. It helps us track —" A flick of Snape's fingers indicated the former Potions Professor didn't need further explanation. "Ron, Neville and I, between us, we've trained with or worked with a lot of Aurors, and a fair few of the ones we trained with ended up as Azkaban guards. I've had a brief opportunity to examine your arm —"

Snape was there before him, and his voice was deadly. "Do you mean to tell me that my freedom depends on the perspicacity of Ronald Weasley and Neville Longbottom?"

"I would have thought you'd have a better opinion of Neville these days," Harry said, "given he killed Nagini."

"The ability to cut the head off a giant snake, whilst admirable, does not speak to qualities of intellect, reasoning, or insight."

"I'm glad you find something to admire in Neville."

Snape regarded him over his clasped hands. "Whatever Neville Longbottom's qualities, capacity for logical thought is not foremost among them."

"Then you'd better hope I can spot the caster myself, hadn't you?" Harry said. "Can I see your arm, Professor?"

Despite their fragile truce, Snape's expression darkened. His thin lips thinned further and the glare he gave Harry could have given a Basilisk a run for its money. "If you're expecting me to be gullible enough to believe all that claptrap about Aurors being able to identify the caster of a spell through some mystical ability, I'm afraid you're destined to be disappointed."

"It is about eighty percent claptrap," Harry agreed cheerfully. "A bit of mystique makes our jobs much easier. And the twenty percent that's true isn't mystical ability, it's a skill that took me some bloody hard work to learn. I can't wave my wand, or go into a trance, and produce a name. What I can do is study a spell, whether it's a charm or a jinx or a curse, and see the little variations that everybody has."

"Like handwriting." Snape sounded as if he was interested despite himself.

Harry nodded. "Very much like. I bet you could do the same with potions, couldn't you? Tell the difference between something brewed by, say, Hermione, and one by Professor Slughorn?"

"Naturally." Snape drew the word out. After another motionless moment studying Harry, he began to unbutton the sleeve of his coat. "If this works, do I have your word that you'll say nothing to Shacklebolt?"

"If it's Azkaban that's worrying you —"

"Do I have your word?" His cuff unbuttoned, Snape made no move to push up his sleeve.

"Alright, then. If Ron, or Neville, or I, can work out who's doing this, I won't tell Kingsley that you're still alive."

Snape inclined his head. "Adequate."

He pushed up his sleeve and then the shirt-sleeve beneath it, and turned his wrist.

Harry gasped with shock. The oval of withered flesh had grown, extending now from Snape's wrist almost to his elbow, and it seemed deeper somehow, as if the curse was eating its way through to the wizard's bones.

"Buggering Boggarts," he breathed. "I thought — that is, Hermione gave me the impression it was getting less severe, but …"

"Yes," Snape said, barely above a murmur. There was no emotion in his voice, nor, when Harry tore his eyes away from the horrifying decaying flesh, in his face. "It seems, Potter, that I will have less time than I expected to teach Granger the basics of her job."