John Doe was an injured Zalkonian whose body was evolving into a being of pure energy, a change his government wanted to suppress.
The crew found a badly injured humanoid in a crashed escape pod in Transfigurations. He had lost his memory, so Dr. Crusher called him John Doe. As he recovered, his body began a strange mutation, and he showed the power to heal others, even reviving Worf after a serious injury.
John Doe proved to be a Zalkonian, one of a few whose bodies were transforming into a higher, energy-based form. The Zalkonian government saw such people as a threat and hunted them. When Commander Sunad tried to seize him, John completed his transformation and departed as a being of light. Mark La Mura played him.