It's Harry's third year at Hogwarts; not only does he have a new "Defense Against the Dark Arts" teacher, but there is also trouble brewing. Convicted murderer Sirius Black has escaped the Wizards' Prison and is coming after Harry.
Dumbledore: A word of caution: dementors are vicious creatures. They will not distinguish between the one they hunt and the one who gets in their way. Therefore I must warn each and every one of you to give them no reason to harm you. It's not in the nature of a dementor to be forgiving. But you know happiness can be found even in the darkest of times, when one only remembers to turn on the light.