"Have you had him this whole time? How did you get him?"

"Percy had him before me. He found him in the garden, and mum let him keep him, and then I got him when Percy got an owl. He always seemed like just a normal rat. I guess it was odd that he lived so long"

"Okay, uh, maybe whatever apparently didn't kill him did hit him really hard on the head?" Cam suggests.

"Maybe. I should go see if--if--" he stands up, jerkily, and starts walking toward the door, not quite steady on his feet.

"Do - you need company -?"

Answering that question would require understanding it and putting words in an order and speaking them, so it's not happening. Marionette-walking out of the library is what's happening.

"I'm... sorry about your rat. He didn't act weird? Just lived a long time?"

"Mostly he just slept a lot. I should, uh, go get him. Or try to get an explanation out of Lupin or something."

"Do you want company or nah?"

"Um, it'd be cool if you came? Scabbers might have gone somewhere else while we were talking. It's fine if you'd rather not though."

"I don't mind." He follows Ron.

So do Harry and Hermione. It's not hard to catch up with Lupin, and he doesn't seem to mind the company.

They go up a couple floors and down a couple hallways and there's that suit of armor, distinctive axe and all.

When Lupin levitates the armor aside, there's the rat behind it, wide-eyed and motionless.

"Peter? It's me. Are you alright?"

"What's the matter? Why didn't you tell anyone you were alive? You were safe--unless . . . unless they switched. Oh Merlin, Peter, just tell me it wasn't you!"

The rat dodges around Lupin's legs and bolts down the hallway, Lupin in hot pursuit a moment later.

"I really wish you'd told me what was going on so I would know whether to help catch him!" Cam says, exasperatedly, following at the briskest walk he can.

Lupin is still not super coherent. He yells "Traitor!" and "Murderer!" and then remembers he's a wizard and starts throwing bolts of red light. A couple of first-year Hufflepuffs emerge from an intersecting corridor, take in the scene, and head back the way they came from. 

Scabbers or Peter or whoever is a small and moving target, but there's not really anywhere to hide in this hallway; soon rat and human skid to a stop, the former unconscious and the latter panting.

"You wanna catch us up? So we know how disturbed to be about the decade as a childhood pet thing, if nothing else?" Cam wonders, when he gets within speaking distance.

Lupin stares at Cam for a bit, blinking like he's trying to remember where he is and how to form sentences. Eventually he starts talking, looking from Cam to the rat on the floor every few seconds as of to reassure himself the latter is still there.

"I can try. It's a long story, and I just found out it's even longer than I believed, but I can tell you what I thought I knew yesterday, at least."

"We were all friends, at Hogwarts, me and Lily and James Potter--well, she was Lily Evans then--and Peter, and--Sirius Black. And when the war came, we all fought together against You-Know-Who. And Lily and James fought as well as anyone, better, they were amazing, and it painted a target on their backs. And then they had Harry."

(Harry and crew have caught up with them, and are listening in variously flavored bewilderment.)

"So they had to go into hiding. Got their house magically concealed, set it up so nobody except Black could have told anyone where they were living. Everyone knows how well that went."

"For those of us who arrived in this universe yesterday afternoon..."

"You-Know-Who showed up at their house and murdered them! And then, and then, what we all heard is that Peter found out Black had betrayed the Potters, tracked him down, and Black murdered him too. But they never found a body, just a finger, and Black laughing like a madman in the ruins, and I don't know what to believe anymore."

"...do you want dioramas of the scene, or - to call the wizard cops, or..."

He twitches a bit at "wizard cops". "I don't think I know enough yet. If you can find out who the secret-keeper was . . . or who killed the twelve muggles when Sirius and Peter fought . . . I don't know if you can, though, will dioramas show who blew up the street?"

"Probably not, or at least not without a lot of very careful inspection of time slices, I can't conjure magic stuff. I don't know if 'secret-keeper' is a conjurable parameter but I can try that -" He holds out his hand.