goblet of fire;

In GoF Moody's identity is taken by Barty Crouch Jr., a death eater who escaped from Azkaban with his parents' help. Crouch attacked Mad-Eye and locked him in his own magical trunk, keeping him alive because he needed periodically some Moody's hair to prepare the polyjuice potion ( a potion that can give you the appearance of the person you want, if you use as an ingredient something from this person, like a chunk of hair). Under Moody's appearance, Crouch helps Voldemort to rise again.

Well, almost at the end of the book, we can see for a moment the true Alastor Moody...:

"The real Mad-Eye Moody was at the staff table now, his wooden leg and his magical eye back in place. He was extremely twitchy, jumping every time someone spoke to him. Harry couldn't blame him; Moodys fear of attack was bound to have been increased by his ten-month imprisonment in his own trunk"

(Goblet of Fire, chapter thirty-seven: The Beginning)