<P> Ron and Hermione's friendship later suffers when Ron believes that Hermione's cat, Crookshanks, ate his rat, Scabbers . At Christmas Harry receives a mysterious present, a late - model Firebolt broom . Fearing it might be cursed, Hermione reports the gift to Professor McGonagall, which leads to more bad feelings between her and Ron and Harry . </P> <P> Ron, Hermione, and Harry join the effort to save Hagrid's hippogriff, Buckbeak, from being executed for attacking Draco Malfoy, after Draco provoked him . Their efforts are unsuccessful, but Scabbers reappears shortly after they hear Buckbeak being executed . </P> <P> Ron chases Scabbers, only to be attacked by a big black dog, which Harry has seen several times before . The dog drags Ron through a tunnel under the Whomping Willow into the Shrieking Shack . Harry and Hermione follow, and there is a brief standoff when they find Ron with Sirius Black, who has transformed from the dog . Lupin enters, and they explain the situation to Harry and his friends: Lupin is a werewolf, which led to his friends James Potter, Sirius Black, and Peter Pettigrew becoming animagi . Lupin explains that Scabbers is Pettigrew in his animal form; he has been hiding from Black, whom he had framed for the murders of Harry's parents and the thirteen Muggles . </P> <P> Snape arrives to apprehend Black but Harry, Ron, and Hermione knock him unconscious . Lupin and Black transform Pettigrew back into human form and prepare to kill him, but they are stopped by Harry, as he feels his father would not have wanted it . He convinces them to give Pettigrew to the Dementors instead . </P>

Summary of harry potter and the prisoner of azkaban in 100 words