No dice.

"Oh, I have a thought. Whoever it was would have had to tell You-Know-Who the location in person, so they would have to have met."

"I'd need a time."

"It would have been not more than a few days before Halloween night in 1981."

"That's a range, we'd wind up spying on his dinner and whatnot. I surmise the wizard justice system failed at this already but does it not have any more precise tools?"

"Not for piecing together things that happened so long ago. And in Black's case . . . you have to understand, the war had ended literally the day before, and it looked so open and shut. He didn't get a trial."

"...well, maybe he should get one now, unless you have some kind of double jeopardy rule about it?"

"That. Seems like a good idea, yes." He looks at the rat again. "I think the next step is to ask Pettigrew some questions." He points his wand and does some wordless spell, and the rat grows and blurs and turns into an unconscious human in damaged black robes. He looks somewhat the worse for wear; most obviously, he's minus a finger.

"...in the hallway? In front of the kids?"

"It's about my parents, isn't it? I want to know."

"Still, yes, not in the hallway would be better." He taps the unconscious man with his wand, turning him almost entirely transparent, leaving a distortion in the air like someone tried to photo-edit him out of the universe and didn't quite manage it. Then Lupin levitates him and starts looking around for the nearest available room.

Cam can't help him there but he can say, "It's Harry's parents and Ron's rat but could you," fluffy kid whose name he still forgets, "perhaps fetch a professor, such as Professor McGonagall?"

Well, her name definitely isn't Cat or she'd be dead of curiosity, from the look of things, but she nods and runs off as Lupin maneuvers Pettigrew into what looks to be a storage room for extra desks.

"Thanks Hermione."

Hermione, that was it. Cam does not conspicuously repeat this to himself.

They're back just as Pettigrew is waking up from the stun.

"Hello again. Miss Granger has explained the latest collection of improbable events." 

Really, this is all her fault for letting the Defense Professor get too close to the Weirdness Magnet. She should never have let them be in the same wing as each other. Except then, apparently, a man who she thought had died a hero's death would still be posing as an animal and living in her student's pocket.

"Yeah, I thought maybe the situation would benefit from context and emotional distance and adulthood all existing in the same person at the same time?"

"Quite," she says, shutting the door and tapping the lock with her wand. "Well, this is certainly Peter Pettigrew. An unregistered animagus?"

"Yes. He learned while we were at Hogwarts."

"I see. And you think he, and not Black, may have been the Potters' secret-keeper?"

"Yes. It's just the sort of clever idea that--that Black would have come up with. And they wouldn't have told--anyone."

Pettigrew has apparently been silent but increasingly conscious; now he shrieks, "Lies! It's all lies! Black was the traitor, not me!"

"Why did you hide, then? Why did you live as a rat for ten years rather than tell anyone you were alive?"

"Because . . . because I was afraid he'd escape from Azkaban and come after me!"

"Don't be ridiculous, nobody's ever escaped from Azkaban."

"P-professor McGonagall--you believe me, don't you? He's trying to frame me--he's a werewolf!"

"Bloody hell, Peter--"

"Silence!"