Harry learns that Sirius Black, a mass murderer and supporter of Voldemort, is after him. The Ministry and Dumbledore set up protection for Harry by using dementors, hooded things which drain out happiness, to hunt down Sirius. Hermione buys a cat named Crookshanks, which likes to hunt down Scabbers, Ron’s rat. Because Hermione has a full schedule, McGonagall secretly lends her a time-turner to help her get to all her classes. Hagrid becomes the professor for Care of Magical creatures. Draco provokes Buckbeak, a hipogriff, in Hagrid’s first class and is attacked. As a result, the Ministry investigates Buckbeak.
Sirius tries to break into the castle and even attacks the portrait guarding Gryffindors common room. Harry has to deal with the fact that the presence of dementors affects him more than the others, and hears the voice of his mother for the first time. It is also this time that Greg and George give him the Marauder’s map, a map of the entrances to the castle and shows the people in it. He uses the map and his invisibility cloak to go to Hogsmeade where he hears for the first time that Sirius was the one who betrayed them to Voldemort and that he was his godfather.
During the Chistmas holidays, Hagrid tells them that Buckbeak was to be taken cared of by the Committee for the Disposal of Dangerous Creatures. Harry also receives a Firebolt, a world class broomstick, as christmas present, with no card in it (but it is really from Sirius) . Hermione tells McGonagall about the broom, who confiscates it to check for jinxes as she suspects that it was given by Sirius.
When terms starts, Lupin gives Harry teaches Harry the patronus charm for the dementors. In the meantime, Scabbers disappears and accuses Crookshanks of eating him. After being checked thoroughly, Harry gets his Firebolt back and uses it during the match against Ravenclaw, where they win spectacularly. They celebrate all through the night in the common room. In the morning, they were all awakened by Ron’s screams who swears that Sirius Black was inside their dormitory with a knife.
Harry’s map is confiscated by Lupin. Nevertheless, Gryffindors wins the quidditch cup.
After their final exams, they meet Fudge who was there to witness the execution of Buckbeak. The three go to Hagrid’s where Hagrid hands Scabbers to Ron. When Dumbledore, the executioner and Fudge arrive to carry out Buckbeaks’s sentence, Hagrid ushers them out. They witness the execution of Buckbeak from afar. Scabbers suddenly runs away and Ron gives chase towards the whomping willow. A big dog appears and drags Ron to an opening in the whomping willow. Harry and Hermione follow and they are surprised to find a passage leading to the Shrieking Shack, an empty hut outside Hogsmeade rumored to be haunted. It turns out that Sirius is an animagus, a wizard that can turn into an animal. Lupin joins the party and together they tell Harry that as students at Hogwarts, they were friends with Harry’s father, together with Peter Pettigrew; Lupin was a werewolf; that as precaution, Lupin was taken to the Shrieking Shack every full moon so that he could transform there; the other three learn how to become animagi so that they could accompany Lupin during these transformations; when Dumbledore learned that Voldemort was hunting the Potters down, Sirius was made Secret-Keeper in a Fidelius charm, a charm that will hide wizards from anyone except the secret-keeper; Sirius changes with Peter in attempt to confuse Voldemort; Peter betrayed them and delivered the Potters to Voldemort; that when Sirius hunted him down, he shouted “how could you” to Sirius, cut his finger off and transformed into a rat; Peter lived with the Weasleys for fear that the Voldemort’s supporters will kill him for causing their master’s downfall. Harry believes them but forbids Lupin and Sirius from killing him as James would not want his friends to become murderers. He suggests that Peter be delivered to the dementors. Snape followed Lupin into the shack and tries to shackle them and deliver to the dementors. The trio stuns Snape and they file out of the shack. When they emerge, Lupin transforms into a werewolf while Peter turns into a rat and escapes. Sirius tries to protect them by turning into a werewolf but the dementors swarm around them. Harry faints and comes around in the hospital wing. They overhear Fudge congratulating Snape for rescuing the trio and apprehending Sirius. Harry and Hermione tries to tell Fudge what really happened. Dumbledore arrives and asks to be left alone with Harry and Hermione. Harry and Hermione tell him what happened but Dumbledore says that it is now beyond his control and that what they need is time. Before leaving, Dumbledore tells Hermione that two turns ought to do the trick and that if they are successful, more than one life will be saved.
Hermione realizes what Dumbledore wants her to use the time-turner. They go back in time, saves Buckbeak and hands him to Sirius so that he could escape. In his anger, Snape ‘accidentally” tells the Slytherins that Lupin is a werewolf which forces Lupin to resign. Dumbledore tells Harry that one day he will be glad that he saved Peter’s life.
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