Harry, Ron, and Hermione decide that Black and Lupin are insane.
Black doesn't seem to care much about that verdict and lunges for Scabbers/Peter to finish the job. Of killing him. In case that wasn't clear.
Lupin tells him to hold up, and says they need to explain things to the kids, especially Harry.
Ron just yells that they're all crazy.
But Hermione channels her inner Jack McCoy and starts up a cross-examination of Lupin, using the power of logic.
Hermione may have found her true calling here.
Lupin tells everyone to gather round for story time and starts to explain everything.
Peter, along with Sirius and James, all became unregistered Animagi while they were attending Hogwarts.
That's one heck of an extra credit assignment.
Lupin explains that the Shack and the Whomping Willow were installed the year he started at Hogwarts. He was secreted out of the castle for his monthly werewolf transformations and hidden away in the Shack so that he couldn't hurt anyone.
The "shrieks" everyone heard were Lupin himself during his painful transformations.
His friends eventually learned the truth and decided to become Animagi to help Lupin. Lupin could be around other animals while he was a werewolf.
Peter became a rat, Sirius became a dog, and James became a stag.
The gang started wandering around in their animal forms and ended up writing the Marauder's Map based on their explorations of Hogwarts.
Hermione calls Lupin out on his group's reckless behavior; wandering around with a werewolf was probably not the best idea.
Lupin admits it was super dumb and says he felt guilty about betraying Dumbledore's trust like that.
And he debated all year whether or not to tell Dumbledore that Sirius Black is an Animagus.
He admits that Snape was right to not trust him.
Sirius perks up at the mention of Snape, and not in a good way. He hates the guy's guts.
Lupin explains to the kids that Sirius played a joke on Snape one time and lured him to the Shrieking Shack while Lupin was in his werewolf form. James pulled Snape back at the last second, or else Lupin would have bitten Snape.
Dumbledore forbade Snape from telling anyone the truth about Lupin. We can see why the guy held a grudge.
And speak of the devil, Snape suddenly appears in the room, holding Harry's Invisibility Cloak.