Harry Potter is about to start his third year at Hogwarts, School of Witchcraft and Wizardry, and can't wait to meet his best friends, Ron and Hermione, again. He lives with his uncle and aunt, who are non-magic people, because evil Lord Voldemort killed his mother and father when Harry was a baby. The rumors say that Voldemort couldn't kill Harry, and lost all his powers while trying to. From then on, Harry Potter became famous in the witches and wizards world, as the boy who destroyed Lord Voldemort.
When he gets to Hogwarts the atmosphere is tense. There's an escaped mass murderer on the loose, who had killed 13 people with a single curse. The only clue for the escaped murderer's, Sirius Black's, whereabouts, was something he said in prison during his sleep: "He's in Hogwarts... He's in Hogwarts..." Harry heard rumors saying that Black was after him and wanted to kill him.
The Dementors, the guards of the prison Azakban, are called to guard the school.
Harry especially enjoys Professor Lupin's class, "Defense against the Dark Arts". Professor Lupin helps Harry to protect himself against the Dementors, who make Harry feel very bad. Professor Trelawney, the divination teacher, keeps predicting Harry's death, when she sees signs of a grim, a black dog, in Harry's tea leaves. In the meantime, Ron awoke at night shouting. He told everybody he saw Sirius Black next to his bed with a knife. One of the teachers, Professor McGonagall, asked the keeper of Harry's dormitory if he saw a man entering the dormitory. He said he did and the man even had the right password. Black disappeared before anyone else saw him.
Harry, Hermione and Ron were walking in the Hogwarts yard, when a big black dog suddenly attacked...