It's a common misconception that the bus featured in the 2004 film 'Harry Potter and the Prisoner of Azkaban' is a Routemaster bus, it is understandable though as the the Routemaster's design owed much to its predecessors. The bus used in the film is in actual fact a RT-class AEC Regent III bus, or to be more precise three RT-class AEC Regent III buses joined together to make the now world famous 'Knight Bus'. 'Knight Bus' is believed to be a play on words by J.K Rowling as the term 'night bus' is synonymous with the late night bus used in Britain at the end of the night and is usually the last bus. Because it is usually the last bus, the term implies refuge or safety because after the last bus there is no other way of returning home. In the film 'Harry Potter and the Prisoner of Azkaban', the triple decker bus was driven by Ernie Prang which doesn't lend itself to much confidence in his driving abilities. The conductor was named Stan Shunpike. The forenames of the drivers come from J.K Rowling's grandfathers.
The general plot of the film centres around the thirteen year old Harry Potter. Harry has endured yet another miserable and disappointing summer at Privet Drive with his relatives the Dursley's. After a harsh comment about Harry's parents by Marge Dursley, the sister of his uncle Vernon, an argument ensues in which Harry 'accidentally' makes Marge inflate and float away. Harry runs away from the house with his luggage and his belongings and catches the Knight bus to Leaky Cauldron where he seeks forgiveness for the previous use of magic outside of Hogwarts school by Minister of Magic Cornelius Fudge.
Harry is joined by his long time friends Ron and Hermione and learns that a dangerous character by the name of Sirius Black has escaped from Azkaban prison and is looking for Harry in order to kill him. Given that Sirius Black is a long time supporter of Voldomort, the man who had killed Harry Potter's parents when he was young, the escape of Sirius Black causes Harry grave concern.