A man locked in an endless struggle with his own antimatter duplicate, whose clash threatened to destroy both universes.
Lazarus appears on a barren planet claiming to be pursuing a monster that destroyed his civilisation. The Enterprise crew gradually discovers that there are two of him, one from the normal universe and one from an antimatter universe, linked through a dimensional corridor. One is obsessed with killing the other, and any meeting between them in the same universe could annihilate both realities.
The calmer Lazarus agrees to a solution that will save everything: Kirk forces the two men into the corridor between universes and destroys the ship that opens it, trapping them in eternal combat. Both were played by Robert Brown.