Harry, Ron and Hermione hear a howl coming from Hagrid. For some reason, Scabbers is going berserk in Ron's pocket. Harry sees the reason; Crookshanks is approaching them. Scabbers escapes from Ron's hands and flees, Crookshanks going after it. Harry, Ron and Hermione follow them. Ron manages to catch Scabbers again. Suddenly a black Dog attacks them, jumping on Harry and rolling off og him. When attacking a second time, Ron pushes Harry aside and is caught by the dog and dragged away. Harry is hit by something. In the dark he lights the scene with a ¤ Lumos and sees the Whomping Willow swinging it's branches at them. The dog drags Ron into a large gap in the base of the trunk of the Whomping Willow. Ron's leg is broken while struggling to keep from being pulled in.
Crookshanks calms the Whomping Willow by placing his paws on a knot on the trunk. They enter the gap into a long tunnel. The tunnel leads far down into a room and Hermione realises it's the Shrieking Shack. They follow the trail of something being dragged and end up in a room where they find Ron.
It's a trap. The dog in fact is an Animagus. It is Sirius Black, closing the door behind them. With a ¤ Expelliarmus he disarms Harry and Hermione. Harry, loaded with hate for Black, lunges at him. A struggle follows. Harry manages to get hold on his wand. Ron and Hermione who are still struggling with Black are told by Harry to get out of the way, and they step aside. On the verge of killing Black, Crookshanks leaps onto Black's chest protecting him. Someone is coming, they hear footsteps. It is Lupin. Entering the room he disarms Harry and Hermione with a ¤ Expelliarmus. Lupin then asks "Where is he, Sirius?". Harry is confused. Lupin seems to suddenly have some insight, asking half-questions to Black. Black appears to confirm with a nod. Lupin helps Black to his feet. Lupin wants to explain but Harry and Hermione are too angry at this apparent betrayal to listen to him. Hermione yells out that she knows Lupin is a Werewolf. She has known ever since Snape had them write an essay about Werewolves. Lupin starts to explain that he knew where Black was because he saw him on the Marauder's Map. He is Moony, one othe the creators of the map. Moony is his nickname from his student days at Hogwarts. He figured they would sneak out to visit Hagrid and the invisibility cloak does not prevent them from showing up in the Marauder's Map. When he saw them leave from Hagrid's cabin on the Map, they were accompanied by someone. Acording to the map, two people were dragged into the Whomping Willow's. He explains to them that Scabbers is not a rat, but a wizard. Scabbers is actually Peter Pettigrew.