Transfigurations

The Enterprise finds a crashed escape pod and rescues its only occupant, a man close to death. He heals astonishingly fast but has no memory, and the crew call him John Doe. His cells are changing in ways she cannot explain, he suffers bouts of pain and begins giving off bursts of energy, and he finds he can use it to heal others, including O'Brien's injured shoulder. A ship arrives from Zalkon, and its commander, Sunad, demands John's return, calling him a criminal. John tries to escape in a shuttle, accidentally kills Worf in the struggle, then brings him back to life. Sunad uses a device that leaves the Enterprise crew unable to breathe, and John heals them all. His memory returns: his people are evolving into beings of energy, and their government kills those who begin to change. John completes his transformation, offers Sunad the same future, which he refuses, and departs as pure energy.