Cat, Rat, Dog

Cat, Rat, Dog is the 17th chapter of Harry Potter and the Prisoner of Azkaban.

Summary
Scabbers runs near the forest and Ron and Neville chase after him, and Lleweyln appear, chasing after the weasel. In their pursuit, the big black dog, Sirius, arrives and jumps Ron, trying to bite at the weasel. Sirius grabs Ron's arm and drags the boy, with Harry trying to stop him. Sirius drags them to the Whomping Willow and drags Ron underneath it. Lleweyln reaches and touches a knot in the tree, causing it to go limp, giving Harry, Hermione and Neville a chance to get to the passage under the tree, leading to the Shrieking Shack. In the tunnel, Harry tells them that the dog was Sirius. Reaching the Shack, Harry finds Sirius arguing with Regulus Black. Sirius tells them that Regulus was not the one who tortured Frank and Alice Longbottom, but Neville refuses to believe him. Remus arrives, and Sirius explains that he believes Regulus, and that Barty Crouch, Jr. Scabbers tries to get away, but Remus uses his wand and Scabbers turns into the man, Barty.

Deviation from Canon

 * Both Regulus Black (Lleweyln) and Barty Crouch, Jr (Scabbers) are Animagi
 * Barty Crouch is introduced earlier than in canon.
 * There is nothing in canon that would suggest Regulus and Barty were friends at Hogwarts.
 * Remus is not involved in helping to hide Regulus and expose the real culprit.
 * Neville is present for this events of this chapter, as oppose to canon.