Marauders

A purple flash of magic extinguishes the fireplace in the common room, and you fall out of thin air onto the carpet, wearing a torn Gryffindor robe. You're clutching the fat old rat in a death grip, wrapped in shining magical chains.

-What the hell? - Sirius pins you harshly to the floor, pressing his wand against your throat. - Who are you?! Where did you come from?

-Peter... Wormtail! - you scream, choking. - He will betray you! He will hand the Potters over to Voldemort and kill you all!

James turns pale, shielding the frightened Peter from 1976:
-What nonsense?! The tail is our brother! We would die for each other, but we would never betray each other!

Remus picks up your fallen tie in shock:
-James, wait... The year of production on the emblem is... 2026. That's impossible...

Sirius presses his wand harder into your throat, leaning down to your ear:
-Speak quickly! How do you know our names and why should I trust a girl from the future?!