Thirteen year-old Harry Potter (Daniel Radcliffe) has reluctantly spent yet another summer with the Dursleys, his dismal relatives, 'behaving himself'? and not practicing any magic. That is, until Uncle Vernon's bullying sister, Aunt Marge (Pam Ferris), comes to visit. Aunt Marge has always been particularly horrible to Harry and this time pushes him so far that he 'accidentally'? causes her to inflate like a monstrous balloon and drift away!
Fearing punishment from his Aunt and Uncle (and repercussions from Hogwarts and the Ministry of Magic, which strictly forbids students from using magic in the non-magic world), Harry escapes into the night.
He is promptly picked up by the Knight Bus, a fantastic triple-decker purple vehicle that whisks him off to the Leaky Cauldron pub. Upon arrival, Harry is met by the Minister of Magic, Cornelius Fudge, who inexplicably doesn't punish the teenager for his errant wizardry and instead insists that he spend the night at the Leaky Cauldron before heading back to Hogwarts for his third year of study.
It quickly transpires that a dangerous and enigmatic wizard, Sirius Black (Gary Oldman), has escaped Azkaban prison and is believed to be searching for Harry. Legend has it that Black was responsible for leading Lord Voldemort to Harry's parents and ultimately to their subsequent deaths; it is also believed that he is determined to kill Harry too.
To make matters worse, Hogwarts is playing host to the Dementors, the terrifying Azkaban guards who are stationed at the school in an attempt to protect the students from Black. The Dementors suck the souls from their victims and, unfortunately for Harry, they seem to have more of an effect on him than the rest of his classmates. Their ominous presence chills the young wizard to the bone, rendering him virtually helpless, until Professor Lupin (DAVID THEWLIS), the new Defense Against the Dark Arts teacher, trains Harry in how to use the Patronus Charm to sh