Here's the breakdown:
* Barty Crouch Jr., the son of the former Head of the Department of Magical Law Enforcement, was serving a life sentence in Azkaban for torturing a group of Muggle-borns.
* He escaped Azkaban with the help of his mother, who used Polyjuice Potion to switch places with him.
* Barty Crouch Jr. then infiltrated the Ministry of Magic and disguised himself as Alastor "Mad-Eye" Moody, the Auror who was meant to be Harry's Defense Against the Dark Arts teacher.
* He murdered his father, Barty Crouch Sr., to keep his escape a secret and ultimately to help Lord Voldemort return to power.
The truth is revealed later in the book, adding to the shocking twists and turns of the story.