1. A Troubled Summer & The Escape of Sirius Black
Harry Potter begins his third year at Hogwarts with another miserable summer at Privet Drive. The Dursleys continue to mistreat him, and he struggles with being unable to use magic outside school.
One evening, Aunt Marge, Uncle Vernon's sister, visits. She insults Harry's parents repeatedly, causing Harry to lose control and accidentally inflate her like a balloon. Panicked, Harry runs away from the Dursleys, believing he will be expelled.
While wandering the streets, he sees a large black dog lurking in the shadows. Before he can react, the Knight Bus appears and picks him up, taking him to The Leaky Cauldron. There, he is greeted by Cornelius Fudge, the Minister of Magic, who surprisingly does not punish Harry. Instead, he warns him to stay in Diagon Alley until school begins.
Harry learns that Sirius Black, a dangerous murderer, has escaped from Azkaban, the wizarding prison. People whisper that Black is after Harry.
2. The Dementors & The Train Ride to Hogwarts
At Diagon Alley, Harry reunites with Ron and Hermione and learns that Ron's pet rat, Scabbers, looks ill. Hermione buys a new pet—a cat named Crookshanks, who immediately starts chasing Scabbers.
Before leaving for Hogwarts, Mr. Weasley pulls Harry aside, warning him that Sirius Black is dangerous and wants to kill him.
On the Hogwarts Express, Harry, Ron, and Hermione sit in a compartment with Professor Remus Lupin, the new Defense Against the Dark Arts teacher.
Midway through the journey, the train stops, and Dementors, the guards of Azkaban, enter to search for Black. One Dementor approaches Harry, making him relive his worst memories—his mother's scream as Voldemort killed her. He passes out.
Lupin repels the Dementor with a Patronus Charm, explaining that Dementors feed on happiness and affect Harry more because he has experienced greater trauma.
3. The Grim Omen & Trelawney's Prediction
At Hogwarts, Dumbledore announces that Dementors will patrol the school. He warns students that Dementors have no mercy and will attack anyone who crosses them.
In Divination class, Professor Trelawney predicts Harry's death, claiming to see "The Grim", a spectral black dog associated with death, in his tea leaves.
Now our main story begins here.....