Professor Quirrell threw away his wand as though it had caught fire, lurched away from Harry, and blurred down into his Animagus form.

Harry pressed his hands over his eyes, waiting for the pain to fade.

We did it. Professor Quirrell has the Stone.

Harry exhaled slowly. Transfiguration was powerful enough already, and now they had the power to make it permanent.

You couldn't Transfigure specific people, of course, and nor could you Transfigure lost books, but bizarrely enough you didn't need perfect knowledge of what you were making to Transfigure it - Professor McGonagall didn't have an encyclopaedic knowledge of porcine biology, and Harry didn't know the precise specifics of a rocket. In fact, even steel was complicated: there were different sorts of steel with different amounts of carbon and other additives. Transfiguration could clearly get information from somewhere that wasn't the caster's brain.

Harry opened his eyes and saw for the first time his surroundings.

The floor was worn and grey with dust, stained oddly brown in places. A slashed painting hung crookedly above a ruined fireplace, cold ashes spilling out over the floor. Nearby was a chair with three legs torn off. The room looked like it had been shredded. The whole place felt uneasy, like the quiet aftermath of a storm.

Harry suppressed a shiver. Ahem, said Hufflepuff, it might actually be a good idea to be afraid of magical haunted houses.

The snake lay unconscious on the dusty floor, a sealed silver pouch behind it. Harry couldn't Innervate Professor Quirrell, and he wasn't sure that it was possible to shake a snake awake. Absently, Harry wondered how the rules for Animagus transformations worked - if you touched the Stone to an Animagus, were they trapped forever? What happened if you turned into an Animagus while holding an indestructible magical artefact, then died?

It was probably a good idea to start planning the best applications of the Stone, so Harry closed his eyes and thought.

Transfiguration seemed even more strange the more that Harry considered it. The universe, or the Source of Magic, or whatever, would obligingly fill in all the necessary information about nerve configurations and biochemistry and such if you said give me a working pig from scratch, but not if you said give me a working Hermione Granger from scratch. It would give you all the information you needed, and which you absolutely didn't already know, to Transfigure a computer, but not to Transfigure a nanofactory. That didn't necessarily mean that nanofactories were impossible - Harry had tried to Transfigure a computer slightly more powerful than modern-day technology, and it hadn't worked either.

Harry had been slightly disturbed by the idea that a fully living pig had popped into being, then vanished. He had wondered what would happen if you tried to Transfigure a generic human like McGonagall had Transfigured a generic pig, but had dismissed the experiment as hideously unethical.

That pig had known how to stand up, so its brain had worked perfectly well. Could it have just been some sort of Platonic ideal pig? Probably not, since it would be perfectly possible to Transfigure, say, a pig with five legs... Had Harry been too dismissive of the idea that souls existed? The existence of Voldemort's Horcruxes was at least weak evidence for them, and it would at least explain why you couldn't Transfigure specific people.

You couldn't Transfigure magical objects, but you could Transfigure something magical into something mundane, so the Stone should cure things like lycanthropy. If magic was based on a marker gene, that probably wouldn't make the wizard into a Muggle.

That was when Harry made the obvious connection.

Wizards were essentially Muggles with magic. They counted as magical creatures, but that was only a genetic thing. It would be entirely theoretically possible to use gene therapy to make a Muggle magical.

You couldn't Transfigure the mundane into the arcane. You could Transfigure all of a Muggle's genes into the exact same boring old chemical DNA, but with the genetic marker for magic - the Blood of Atlantis.

That would at a stroke extend people's lives massively, make them much, much safer... There were bound to be problems, but the number of lives saved would be enormous.

There was no reason the Universe couldn't contain "I-win" buttons like the Philosopher's Stone.

Harry began to grin as the enormity of what had just happened hit him. He'd need to talk it over with Professor Quirrell, but things were looking good for world optimisation.

Harry opened his eyes and looked up.

He was looking at the tip of an outstretched wand.

The wand fell away. "Harry?" the man whispered.

Bellatrix? was Harry's first, absurd thought. The man before Harry was tall and once-handsome, his beard unshaven and his hair long, but his eyes were dark and hollow, and he looked prematurely old, and thin - as though he'd been emaciated once, and never quite recovered. Yet his wand was perfectly steady, and he bore an air of danger and power that Harry had come to associate with powerful wizards.

And that was when Harry's brain finally made the connection, the conversation with Hermione that felt a lifetime distant. Harry looked directly into the eyes of Sirius Black.

Do not react. You are an innocent little boy who has never seen a picture of Sirius Black before.

"Who- who are you?" Harry asked, tingeing his voice with nervousness.

"Oh, Harry," said the man, looking at him with strange, sad fondness. A note of confusion coloured Harry's mind. "You look just exactly like your father. But with your mother's eyes..."

Black trailed off, looking exceptionally uncomfortable, like he wanted to say something but had no idea how to. He sat down heavily in a padded chair that hadn't been there a moment ago.

"You knew my father?"

Black pressed his bony hands over his eyes.

"Oh yes. It was my fault, Harry, I'm sorry, I as good as killed them..."

Can we ever have a conversation that's not cryptic? asked Gryffindor.

Black took a deep breath. "My name is Sirius Black. I was your father's best friend, and he named me your godfather."

Within himself, Harry barely flinched at all. His parents hadn't been perfect, hadn't been omniscient, he knew that well enough. On the outside, he looked appropriately horrified.

Black winced. "I know, I know what they all think about me. I didn't do it, Harry, not deliberately." He pulled up his sleeve, revealing a bare arm. "I wasn't the one who showed Voldemort where your parents were hiding." His voice hardened. "It was Peter."

"Um," said Harry, "what?" Pettigrew was the one almost all the conspiracy theories blamed... it wasn't wrong just because the conspiracy theorists said it, in fact, if anything it was very weak evidence that Pettigrew really was to blame, but even so...

Black sighed, and conjured a chair for Harry. "Peter is a Metamorphmagus, he can change how he looks, and he used that in the war. He was working as a spy, and he turned it against us." In the dim, non-magical lamplight, Black's skin looked grey. "He came to me, asked where James and Lily were hiding. Please, Harry, understand that Peter and I had been friends for years. I thought he would have died under the Cruciatus before telling Voldemort anything. Like a fool, I told him, and..."

Black's voice trailed off.

OK, could this be genuine?

Complexity penalty, said Ravenclaw. The problem is that actors this good are rare... probably not that rare amongst Death Eaters, though...

"So, um, Mr. Black, how did you end up being blamed, then?" Innocent-Harry enquired tentatively.

"Just Sirius, Harry." Sirius withdrew a bar of Honeydukes' chocolate from his pocket and offered some of it it to Harry, who refused politely.

"James and Lily were killed, and I..." Black seemed to be struggling to get the words out.

"James and his family, they took me in - my own House disowned me because I wasn't stupid enough to hate Muggleborns. And Lily brought out the best in James, once he'd grown up a bit, she was like a sister to all of us. I don't know what else you've heard about them, Harry, but they really were some of the best people you could hope to meet."

Sirius took a bite of chocolate.

Harry was feeling strangely warm, like he hadn't quite felt since that day in the hallway with Severus. I'm tentatively saying genuine, said Slytherin. Perfect Occlumens...es? Occlumentes? Are rare, and this doesn't sound like a lie.

Motivated cognition, said the Inner Critic.

"Peter came to me, disguised as an Auror I knew fairly well. He said he'd tracked Peter to a Muggle neighbourhood. I was too angry and hurt to think straight, and I followed him. He'd kidnapped someone, fed them Polyjuice to look like Peter, bewitched them to shout about how I was a traitor. We were both Disillusioned, and he changed into me, dropped his Charm, and blew half the street apart before I had time to think."

"How did the real Peter escape, then?" asked Harry sceptically.

Sirius stood up abruptly and blurred into a large black dog, then back again. "Your father, Peter and I were all Animagi." Sirius' expression grew distant, and wistful. "Have you met Remus, Harry?"

Harry nodded.

Sirius looked carefully at him. "Remus is a werewolf."

"I see," said Harry. That explained a lot, actually - Mr. Lupin was a skilled wizard, and yet his robes were shabby and he drifted from one job to another; those scars on his face never faded. Werewolves were deadly even to some adult wizards, and unable to control their actions once transformed. They were also entirely harmless as long as they took Wolfsbane Potion once a month, which was reasonably cheap, and yet the Ministry didn't provide it for free to werewolves because people were stupid.

"We found that out about him by the end of our first year. Wolfsbane didn't exist at the time, so Dumbledore had put measures in place for him - the Hogwarts wards tracked him, and every month he came to this place," Sirius gestured around, "to transform safely."

Harry raised his eyebrows. "We're near Hogwarts?"

"Yes. I don't know how your scaly little friend over there knew about it, but this is the Shrieking Shack in Hogsmeade. There's a loophole in the protective enchantments on Hogwarts. You can Portkey out through the Shack, but not in."

That made sense. Hogwarts' grounds were big - it had all happened too quickly to notice at the time, but now that Harry looked back they'd come nowhere near the boundary of the wards.

"Remus used to keep all sorts of things as Portkeys - twigs, combs, , and he always panicked when we asked what they were." Sirius smiled fondly. "He honestly thought we'd abandon him, the prat. Instead, the three of us learned to become Animagi - your father was a stag, Harry. Werewolves aren't very aggressive towards other animals, so that way we could keep him company, and be an extra layer of security. We started calling ourselves the Marauders, the greatest pranksters Hogwarts had ever seen."

Sirius's eyes were sparkling. "Speaking of which, I heard about your little escapade with those bullies. Not bad, young Prongslet, but it doesn't measure up to the one where we got McGonagall to- well, I'll tell you another time."

The smile vanished as though it had never been. "Anyway. Peter was a rat, which suited him down to the ground. When he blew the street apart and framed me, he changed forms and ran away just as the Aurors arrived."

Sirius smirked bitterly. "It was the second-best prank I've ever seen."

Silence reigned while Harry tried to digest all of that. It... didn't entirely sound like something a real Death Eater would say, it was too complex, a hardened Dark Wizard would surely just claim they'd been Imperiused or tortured or something. That said, it had pulled his heartstrings perfectly, which was convenient.

Something in Harry felt repulsed at that thought, and it occurred to him again that that might be how he spent the rest of his life - never quite convinced that a smile was just a smile. Harry glanced over at Professor Quirrell. Harry had inherited Riddle's thought patterns and some fragments of memory. Quirrell had inherited an input of Riddle's entire mind. In that moment, Harry thought he might have begun to understand how Monroe had felt on that cold October night.

"So our magnificent justice system threw me into Azkaban without a trial. They never even stopped to wonder why Lucius stood by and let that one happen. And then, one day, while I was sitting there happily losing my mind, Dumbledore broke me out."

OK, this is just silly, said Slytherin. It was plausible until now, but come on. I mean, conspiracy theories are all very well, but this is... what next? Were they rescued by, by Draco Malfoy in a Boeing 747? Did they melt through the steel walls of Azkaban with the jet fuel?

When most people try to make things up, it doesn't sound like this, said Ravenclaw. And unlikely things do happen. Shuffle a pack of cards, and it's probable that that permutation has never happened before...

"Why would Dumbledore do that? How would he even know you were innocent?" asked Harry.

"Apparently, Peter, the idiot, came to Hogwarts in his Animagus form, searching for some cursed object old snake-face had apparently hidden there. Peter was never very powerful. He might have been able to run away and live in another country, but it must have been a bit of a disappointment. He went from a warrior for the Dark Lord to a rat on the run, and he obviously decided to go back to his old ways."

Sirius grinned, a curiously canine expression. "Sadly for him, the Marauders' legacy was alive and well. Your friends the Weasley twins happened to show a lovely little map we'd all made to their big brother Bill, who noticed that his brother's rat was called Peter Pettigrew, and ran to Dumbledore. A couple of Memory Charms later, and Bill was in St. Mungo's, Peter was Confunded to turn into me, and I was free."

I quit, said Ravenclaw.

Harry put his head in his hands. They really ought to start paying me more for all this.

It had been horrible to do that to Bill... nothing like as bad as leaving someone in Azkaban, certainly, and Bill had recovered fully and, the twins had said, was now a curse-breaker for Gringotts.

That seemed like an unusually happy ending, and Dumbledore had shown every indication of caring for his students. Could the entire story of Bill's psychosis be pure fabrication? That should get a complexity penalty, but Harry wasn't even sure that was valid when Dumbledore was involved.

Sirius had been remarkably straightforward with Harry for an adult, and at that thought Harry's respect for him rose several notches. "Sirius... that whole story sounds..."

Peter Pettigrew, by all accounts, had been smart. He wouldn't risk everything to run back into enemy territory - literally into the enemy castle - just to try to get some mysterious thing. On the other hand, Voldemort might well have left orders for his servants to try to retrieve Horcruxes after a given interval of time, just in case his creation didn't quite work as planned. Peter might have further supposed that some other Death Eaters had been given similar orders, and feared Voldemort's retribution if Peter had disobeyed orders...

Sirius gave a bark of laughter. "You're telling me, Harry."

Harry rubbed his forehead, then remembered that he was having a conversation with a man who was either a mass-murdering traitor or his caring godfather. "What have you been doing with yourself?"

A sigh. "Well, the war was done, and Moony- sorry, Remus was the only friend I had left in the world, and he'd spent years thinking I was evil. I knew he'd never forgive me anyway, for what I told Peter." Sirius looked down at his robes.

Harry was about to start talking about egocentric bias and how Remus Lupin wasn't the kind of person who would hold a grudge against someone for one stupid mistake, but then decided to shut up.

Sirius collected himself. "I ended up drifting back to Hogsmeade, and took over Honeydukes as 'Ambrosius Flume.'" Sirius gave an elaborate bow. "Sirius Black, ex-convict extraordinaire, mass-murdering Death Eater and purveyor of delicious sweets to children."

Despite himself, Harry snorted.

OK. This is all starting to look like some kind of bizarre plot, said Slytherin.

Who could possibly benefit? asked Ravenclaw. And why make it this complex? And this... well, this?

That story just doesn't sound right. Pettigrew's plot was complicated and needed extremely precise timing, but it went off without a hitch. It's... it looks like what Professor Quirrell arranged in the corridor. It looks set up.

Harry shook his head, as though to rid himself of the thought. "What are you doing here tonight?"

Sirius shrugged. "I live in Hogsmeade. The wards I put on this place when I was in school triggered for the first time in years, and I was curious."

Who could possibly benefit from Peter Pettigrew framing Sirius Black, then being captured himself, then breaking Sirius out of Azkaban? asked Ravenclaw.

What happened as a result of that? mused Slytherin.

Anyone behind this whole thing would have to be unbelievably good, or cheating somehow. Suppose somebody needed Sirius completely out of the way temporarily, for some reason, but still had use for him.

What happened with Sirius out of the way?

Well, Sirius would have been Harry's legal guardian, and in his absence Dumbledore had become Harry's guardian.

Dumbledore, who had given Harry to his parents.

Dumbledore, who "had some sort of foreknowledge".

Dumbledore, who was more than powerful enough to fake the whole thing, to make the scheme work by brute force.

Dumbledore, who had a taste for bizarre, incomprehensible plots.

Dumbledore, who understood the phoenix's price more than well enough to leave a friend in Azkaban for a few years.

Harry stayed silent for a moment.

"Look," said Sirius. "I know it sounds strange, and you're probably right not to trust me. Merlin knows your parents shouldn't have. But I'm not a Death Eater, Harry, not in a million years. Expecto Patronum."

A silver dog burst from the end of Sirius's wand, tail wagging.

Genuine or not, we still shouldn't trust him, murmured Slytherin.

Harry rubbed his temples in a circular motion. He wasn't in any immediate danger. He could afford to take a minute to be tired and confused and elated and horrified and not thinking straight.

Harry was starting to wonder if the universe was deliberately messing with him specifically.

"Good evening," came the dry voice of Professor Quirrell.

Author's Note:

A/N: clues about the Mirror puzzle, which I now realise was probably much too hard:

1. Even Quirrell can't fool the Mirror. It's something he genuinely wants to do.

2. It's something to do with the fact that the Stone (unlike in HPMOR) can perform otherwise-impossible feats of healing, up to and including, for instance, healing the Longbottoms.

3. It involves a backdoor Dumbledore programmed into the Mirror, in case he or an ally, for whatever reason, desperately needed the Stone for some specific purpose. Dumbledore doesn't know what the Stone does. He has, however, read enough books to not seal away the Stone completely, especially when it comes to the most salient purpose that he has in mind.