Earlier that morning …

"Are you sure about this?" Harry's voice was so quiet Hermione had to strain to hear it.

"I believe my confidence that I can teach a first-year potions class is not entirely unwarranted, Potter." That was, unmistakably, Snape. "Granger needs to rest."

"As Hermione, though?" Harry sounded doubtful.

Teach as me? Hermione struggled to climb up from the well of sleep to tell them what a mental idea that was, but exhaustion held her fast.

"Why would they suspect?"

Because I showed them Polyjuice on their first day, you dunderhead. Memorably, if I say so myself. She managed to open her eyes.

Appallingly, the sun streaming through the window showed it was already midmorning. She scrambled out of bed. I was dreaming, that's all … surely? A mad dream about Snape and Polyjuice.

Between them, they'd have enough sense to think of Harry popping down to the class, saying I was delayed and the students had an unexpected free period as a result.

Wouldn't they?

She showered and dressed in record speed, flung her teaching robe around her shoulders and headed for the dungeons at a brisk walk that was only just not an outright run. She had almost reached the staircase when the small and solid figure of Maisie Wilkins bolted out of it and bowled her over.

Hermione landed on her back on the floor, Maisie on top of her. "Help!" Maisie gasped.

"In the classroom —" Michael Rowland panted, appearing behind her.

Colin Aitkins completed the trio. "Stealing the Key!"

So, not enough sense, then. "My teaching assistant," Hermione said briskly. "Who has rather an odd sense of humour, sometimes. Wilkins, do get off me."

Maisie scrambled up. "But —"

Over Colin's shoulder, Hermione had the disconcerting sight of seeing herself, wand in hand and most un-Hermione expression on her face, whip into view around the spiral of the staircase and stop dead. Snape. She glared at him, and miraculously, he seemed to take the hint, moving noiselessly backwards until he was out of sight.

She gave him a slow count of three and then turned to the children. "Come on, you lot, back to the classroom, and tell me what happened on the way."

The story tumbled out of them, mostly with them all talking at once, but Hermione managed to get the gist by the time they reached the classroom door.

"Well done on the charms, Mr Rowland, and for keeping your head. Five points to Ravenclaw. Badly done on a complete lack of foresight, Miss Wilkins. If you're ever in a position where you really do spot a powerful witch or wizard bent on doing harm in disguise, it's generally best not to let on to them you've rumbled them. Five points from Hufflepuff. And for your information, Hogwarts is very well protected. No-one who means you harm could possibly get in." Unless invited, the way Quirrell was, and Barty Crouch Junior, and … best not to mention any of that to the students, though. "Now, in you go."

Snape had wisely not returned to the classroom. Hermione spent a busy fifteen minutes reassuring her students, cleaning up several disastrous attempts to brew Cinderjuice potion — what was he thinking? — and telling several outright lies about the identity of her teaching assistant.

"As I said he's disfigured," Hermione repeated to her first year class. "That's why you'll never see him, or what he really looks like." She would have preferred to claim her teaching assistant was female, to get them off the track of possibly thinking of just which man could take over a Potions class, but Maisie already saw Severus, on the Boggart day. "No, I didn't say what his name was, and I'm not about to. He prefers his privacy. Mr Aitkins, if your hand is up because you have more questions about this morning, you may as well lower it again." Colin did so, expression crestfallen. "Look, all of you. I'm sorry about the disruption this morning. My teaching assistant has a peculiar sense of humour at times. Class dismissed."

As soon as the corridor cleared of students making the most of a fifteen-minute early mark, she stormed down it to Snape's door and hammered on it. When it opened, she hitched her teaching robe more securely over her shoulders and stomped down the hall to Snape's sitting room.

Harry was already there, leaning casually against the mantle-piece. The Polyjuice had worn off and Snape, his usual self, sat in his armchair.

"—safely back under lock and key," Harry said as Hermione came through the door. "Hello, Hermione."

Snape raised an eyebrow. "I see you survived your assault by hysterical eleven-year-olds?"

Hermione put her hands on her hips. "Have you both lost your minds? In what possible universe is Severus Snape disguising himself as me and teaching first year Potions a good idea?"

"We had actual Professor Snape teaching us for six years, and we turned out alright," Harry pointed out.

"They thought you were going to kill them!" Hermione told Snape. "They were literally running for their lives when they crashed into me! They're traumatised! Probably scarred for life!"

Harry smiled. "Like we were after the Troll? Not to mention the Basilisk, the —"

"You are not taking this seriously!" Hermione snapped.

"Oh, come on, Hermione." Harry grinned at her. "From what Professor Snape said, they did very well. Spotted the substitution, cast a couple of appropriate spells, and got away."

Hermione was angry enough to stamp her foot. "That is not the point! They're my students, and I'm responsible for them, and I won't have them bullied and belittled and terrified, not to mention given a Potion to brew that they're not remotely ready for." She threw up her hands. "It would be one thing if I could approve of your approach to teaching, Severus, it's quite another —"

Snape rose to his feet and drew himself up to his full height. "I have been teaching Potions since before you were born," he hissed. "I do not require your approval."

"If you want to set foot in my classroom again, you do," Hermione said firmly. Because, really, someone as devious and as logical as Snape would have come up with a plan that didn't put him to that much trouble if he hadn't really wanted to teach that class.

"Unlikely to arise," he said coldly. "Since my only reason to do so was to prevent the children from inflicting injuries on themselves or others in your absence —"

"And you just happened to have Polyjuice prepared?"

"I happen to have a great many things prepared, Granger, particularly those potions whose brewing takes considerable time." He raised his eyebrows. "Is this the Muggle way of expressing gratitude?" he drawled. "Because I must inform you, Granger, it falls rather short by wizarding standards. I went to considerable trouble to relieve you of the burden of your first class, and —"

"Now look here," Hermione heard herself say, and stopped. Sentences that start with 'now look here' tend to end badly.

And I suppose that if I'd been stuck down here for five years, I'd be ready to jump at the opportunity for a little human interaction, even in a classroom.

Not that Severus Snape would admit such a thing in a million years.

"I appreciate that," she said as mildly as she could. "And of course I'm very grateful for the trouble, and the effort, you took last night, helping me. I don't mind admitting it would be handy to have someone to share the teaching occasionally. And the marking. But don't you see, you can't send them running in terror —"

"I didn't anticipate they'd perceive the deception," Snape admitted, although, being Snape, he managed to make it sound as if it had been the students' fault they'd noticed Professor Granger wasn't Hermione.

"Besides, it's not the first Hogwarts class taught by a Polyjuiced pretender, is it?" Harry asked cheerfully.

"Harry Potter, Barty Crouch Junior is not a good example for anything! He was a Death Eater planning to have you killed."

"Point," Harry conceded.

Hermione levelled her finger at Snape. "I've told the class that my teaching assistant has a horrible facial disfigurement and a bizarre sense of humour." Harry laughed and Snape glared at him. "Which patches things for now, but no more Polyjuice. There can't be two Professor Grangers running around Hogwarts." He gave her a slight nod of acquiescence. "I'm not saying never, but you'll have to show me I can trust you with my students before I even consider letting you take a few classes again."

Snape took the bait. "And how exactly do you propose I do that?"

I knew it! He really did do it because he missed teaching, at least a little. "My marking," Hermione told him. "You can start doing some again — properly. The way I would."

He raised his eyebrows. "While I have previously expressed willingness to assist you with your marking until you manage to acquire appropriate efficiency, I will not waste my time making, as you phrased them, helpful remarks on ill-informed screeds."

"Helpful remarks, or no teaching," Hermione said. It was a good exit line, and she made the most of it, sweeping from the room with a Snape-like swish of her robe — slightly ruined by tripping over the hem of it in the hallway, but at least it was out of Snape's sight.

Not Harry's, who'd followed her. He caught her arm and steadied her. "I don't think they're really all that emotionally scarred, you know. I mean, according to Professor Snape, the three of them have already been to the Forbidden Forest at night and captured a Boggart. I'm guessing they're pretty hard to traumatise."

Hermione rolled her eyes. "Not the point, Harry." She led the way back to her classroom, and waited until the door was closed. "Letting him loose on first year students, I mean, he was bad enough to us and that was before he went through Voldemort coming back, and being a spy, and then being in hiding with only two people alive to talk to for five years. Honestly, Harry!"

Harry shrugged, taking one of the student's stools. "He suggested it. I sort of felt I owed him the chance." He tilted his head a little. "And that you did, too."

"Oh." Hermione felt her anger ebb away a little. She leaned against the bench next to him. "I do owe him, I know, and it's obvious he wanted to do it, he must be horribly lonely by now, but still, Harry. What if one of them had tried to cast an offensive spell instead of running away?"

Harry grinned at her. "You're worried about Severus Snape being hexed by an eleven-year-old?"

"I'm worried about what a man with a filthy temper and a war veteran's reflexes would do to that eleven-year-old," Hermione retorted.

"Give him more credit," Harry suggested. "I saw him fight McGonagall and Flitwick, the night we came back to Hogwarts. He was fighting for his life, and we know now he was fighting to stay alive long enough to give me Dumbledore's message. He still didn't even try to hurt them. He'd Expelliarmus a student who tried to hex him in a heartbeat, but Snape would never hurt a child. Besides …" He hesitated a moment. "He's in pretty good control of his magic, you know. Last night was convincing proof."

"I know it was difficult," Hermione said. "I could tell — he looked completely exhausted."

"It took a lot of power," Harry agreed. "But more than that — I don't know exactly how to explain it. Curse-breaking is … it's like doing a puzzle. Or undoing a knot. One that will remake itself if you hesitate even a second before you pull on the next thread. It takes absolute focus. I don't think there's anyone I've ever seen who could have done a better job of it than Professor Snape."

Hermione bit her lip. "You know what that means, don't you? If he can't break the curse on him, there's no-one else who could, either."

Harry nodded. "The only way is to find who's doing it, and persuade them to lift it."

"What if they won't?" Hermione asked.

Harry's mouth set in a grim line that, for a moment, made him look so much like his seventeen-year-old self that Hermione could almost smell the fallen leaves in the Forest of Dean and feel the chill in the air. "Then I think one more Imperius in a good cause won't do my soul any irreparable harm."

Hermione put her hand on his arm. "Harry — they'll send you to Azkaban for that. It's not the War anymore."

"I think saving a life will be a pretty good defence," Harry said. He shrugged a little, looking down at his feet. "And I'll take the risk. Don't tell Professor Snape, though."

"He's got a right to know —"

"Hermione, he'll never let my mum's son risk Azkaban. He'll insist on doing it himself, and they will sentence him for it. I'm the Boy Who Lived. He's a former Death Eater with only my word for it that he wasn't, really. If he uses one of the Unforgivables …" He shrugged again. "There's no way Kingsley can let that go."

"Now you're making Severus one of your causes," Hermione said.

"Well, if I didn't owe him enough five years ago, I certainly owe him now." He put his arm around her shoulders. "I'm waiting on an owl from a colleague with the list of potential suspects. There must be someone I've forgotten about, because I'm sure the magic is something I've seen before." He sighed. "But it's got a really strange left-handed twist to it that I can't imagine I'd forget."

"Could it be a new curse, one you haven't seen before?"

Harry shook his head. "Apart from the handwriting, it's fairly bog-standard. Straight out of Dark Spells for Desperate Days."

"Then maybe that's because it's being cast the way it is?" Hermione suggested. "I mean, you've never seen a Killing Curse cast on a Protean Charm that's also a tattoo, have you?"

"Maybe." Harry took off his glasses and rubbed his eyes. "Hard to know what effect that has, though."

"Not really," Hermione said. "I mean, we know all the elements, don't we?" She ticked them off on her fingers. "Killing Curse, Protean Charm, Blood Magic. There's got to be a limited number of ways to combine the relevant parts of them." She stood up. "I'll get working on it this evening."

"You should probably take it easy for a few days," Harry objected.

Hermione grinned at him. "I will. Remember — Snape's going to be doing my marking."

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Author's note: I don't know if Blood Magic is really a thing in Harry Potter canon, but blood certainly has some power in canon — for example, Voldemort can touch Harry after his resurrection because it was Harry's blood that was in the resurrection spell, and Lily Potter's protection continues after Harry 'dies' because his blood is still alive in Voldemort. So I've just sort of run with it a bit.

Only two chapters before we find out just who is cursing Snape! Any theories?