In "The Pain in the Heart", the final episode of Season 3, Zack receives third-degree burns and massive tissue damage on both hands after an explosion in the lab. It is later revealed he was working as the apprentice of the Gormogon, a cannibalistic serial killer, and the explosion was designed as a distraction so Gormogon could break into the lab and steal back the silver skeleton. Zack's weaker personality was easily manipulated by Gormogon so he believed his belief system irrefutable, even going as far as to divide the bones of a lobbyist among the 10,000 skeletons in the Jeffersonian's bone storage room, called Limbo. However, Zack still maintained loyalty to his friends. As pointed out by Brennan, he was willing to injure himself to keep Hodgins safe. He gives up the location of Gormogon's house after Brennan makes him realize his logic is faulty.
