Moody received a plea for help from his old mentor in the Order of the Phoenix, Albus Dumbledore. Not only did Dumbledore need another new Defense Against the Dark Arts teacher, but Lord Voldemort had returned, and an experienced presence to protect the students - particularly one Harry Potter - was needed. Moody was the man chosen by Dumbledore to fulfill both roles and he agreed to take the task on for a year.
