During summer holidays in 1993, Alex Deuyour sends a birthday card to his friend Harry Potter and learns that the Weasleys are in Egypt visiting Ron Weasley's brother, Bill, who works as a Curse-Breaker at Gringotts. Alex and his mother, Desiree, learn that Sirius Black has escaped Azkaban. Secretly, they are ecstatic despite Minister Fudge wanting Desiree to capture Sirius, her older brother, and she reluctantly agrees. Meanwhile, Desiree has to return to the United Kingdom, following on a report that Harry lost his temper and used his magic on his uncle's sister, Marge. Alex c
alls out the Dursleys for their treatment of Harry.
Desiree and Alex spend the rest of the summer at the Leaky Cauldron, where they reunite with Harry and on the last night, the Weasleys, Hermione Granger, Kate Nguyen, and Matt Crocitto. Harry is concerned after eavesdropping on a conversation between Arthur and Molly and learns that Sirius has supposedly escaped Azkaban to kill Harry, but Alex tells him the truth.
At Hogwarts, the guards of Azkaban, known as the Dementors, patrol the school looking for Black. Alex and Harry are unusually affected by their presence, and ask help from Remus Lupin, the new Defence Against the Dark Arts teacher, in learning to repel them. Lupin teaches Alex and Harry the incredibly difficult Patronus charm, but Alex and Harry are able to produce full-bodied Patronuses to fight the Dementors. Meanwhile, Black is reported to have been sighted in the village of Hogsmeade, close to Hogwarts.
Harry is given a secret map by Fred and George Weasley, which reveals everyone inside the castle and its grounds. Harry uses it to sneak into Hogsmeade without permission. Hiding inside a pub, he overhears a conversation in which he learns that Black is actually his godfather, and that he killed his own best friend, Peter Pettigrew, while also betraying Harry's father, James, to Voldemort.
Alex tells the group that he and his mother do not believe that Sirius betrayed James and that he is innocent, though Harry, Ron, and Hermione do not believe this.
Alex, Harry, Ron, Hermione, Kate, and Att visit Hagrid at his hut, and are surprised to find Ron's missing pet rat, Scabbers, hiding there. As they walk back to the castle with Scabbers, Ron is attacked by a large dog. The dog drags Ron into a secret tunnel and disappears, coming out on the other side, inside the Shrieking Shack in Hogsmeade. The dog is revealed to be Black in his Animagus form.
Alex, Harry, Hermione, Kate, and Matt reach the Shack, followed by Lupin and Alex's pet pit bull, Bandit, the former of whom spots them on Harry's map. Lupin disarms Harry, then explains why he believes Black to be innocent. Black claims that the crimes attributed to him were actually committed by Pettigrew, who escaped suspicion by faking his own death. Pettigrew is also an Animagus, and has been hiding as Ron's rat Scabbers this whole time. Harry suggests handing Pettigrew over to the Dementors. Alex embraces Sirius, who reveals to the shocked group is also his godfather and maternal uncle, as he is his mother's older brother.
While walking back to the castle, the full moon suddenly emerges and Lupin, secretly a werewolf, starts transforming. Black turns into the dog to protect the others, and Pettigrew escapes amid the chaos. Alex, Harry, Hermione, and Black are accosted by Dementors, and Black ends up being captured.
While Black is held captive inside a tower in Hogwarts, Dumbledore confirms with Alex, Harry, Hermione, Kate, and Matt that he is indeed innocent. He instructs them to save Black, by going back in time and setting him free, before he is handed over to the Dementors. Using a Time Turner, which Hermione has had since the beginning of the year to get to all her classes, they travel back a few hours. Present-day Alex and Harry save their past selves, and the past versions of Hermione and Black from Dementors using a Patronus. They then allow Black to be captured by Snape, before using Hagrid's hippogriff, Buckbeak, to release him from the tower and escape.
The next day, Lupin is forced to resign as his secret identity as a werewolf is now known to the school. Harry is upset at not spending more time with his godfather, Black, but Dumbledore asks him to take comfort in having saved his life. On the train journey back to London, Harry receives a letter from Black, who also sends a replacement pet owl for Ron. After they get off the train, Sirius reveals that he is now living with Alex and Desiree, but when guests who do not know of his innocence come over, he will have to stay in Animagus form. Sirius and Bandit bond and play with each other in animal form.