Harry Potter Knight Bus If you have seen Harry Potter and the Prisoner of Azkaban, then you will remember The Knight Bus. This triple-decker purple bus provides safe and fast transport for stranded witches and wizards. The driver of the Knight Bus, Ernie gives you the keys of the bus. Drive around the streets of London by using Arrow keys, press Space bar to perform a fast U-turn. The green target appears to identify a potential passenger, stop on them to pick them up. The purple target appears to show their destination.