If Remus has been sorted into Ravenclaw.
Third week of school, 1971, Potions…
"Did you hear Snivelly yesterday? Mocking some bloke from Ravenclaw for being a Half-blood, as if he's any different," Sirius mutters as he, James, and Peter work on their potion.
"We ought to get him back somehow," James says, the others nod. Then Snape's potion explodes. The three start laughing instantly.
"Potter, Black, Pettigrew!" Slughorn calls.
"It wasn't us!" James immediately denies, it's not very convincing, given how much they are laughing.
"Oh really?"
"Sir, if I may," A voice says from the desk beside them, they turn to look.
"That's the half-blood I was talking about," Sirius says quietly.
"Yes, Mr Lupin?"
"The three of them were helping me with my potion, you know I'm not very good you see and they always do well. My point is, they have been the whole lesson, it couldn't have been them."
They hear the bell go, Slughorn nods, "Very well, thank you Mr Lupin. Five points to Gryffindor for each of you for helping your classmate." The three high-five.
As everyone packs up, the Marauders turn to Lupin.
"Hey, thanks for giving us an alibi," Sirius tells him.
Lupin smirks, "I wasn't giving you an alibi." The three look confused, "I was giving me one." With that, he turns and walks out of the classroom. The three boys look gobsmacked in his wake, then James smiles.
"Lads, I think we've found the fourth Marauder."