Chang was a telepathic alien who was sent to Earth to acquire human subjects for experimentation. He used advanced alien science to pose as a stage magician, his greatest trick being a disappearing act in which a volunteer would vanish while inside a glass cabinet. This was actually a matter transference chamber which sent the subject to Chang's home world. He was careful to choose only those who had no close family or friends.
The Third Doctor observed one of Chang's performances, then confronted him backstage. Chang then attacked the Doctor, and in the struggle the matter transference chamber was destroyed, stranding Chang. Enraged, he tried to kill the Doctor, but accidentally electrocuted himself on a power cable. (PROSE: And Now for My Next Trick...)