AllMovie

While doing a routine survey of a seemingly uninhabited planet, the Enterprise is suddenly hit by an inexplicable disruption of physical laws, gravity disappearing and the mass of everything within Spock's sensors suddenly dropping to zero. Captain Kirk is informed that the phenomenon extended throughout the known galaxy and far beyond, but was centered on the planet the Enterprise is orbiting, which suddenly has a single inhabitant. Ordered to investigate, Kirk discovers the man to be Lazarus, according to him the sole survivor of his race, who claims to be in battle against a hideous monster. As he seems to be a raving lunatic much of the time and there is no evidence of any "monster," Kirk is inclined to disbelieve him, except that the same disruption in the fabric of the universe reappears, centered on Lazarus, who seems to change personalities from moment to moment. Kirk and Spock eventually sort out the truth, that Lazarus is really two different people, one from a matter universe and one from an anti-matter universe, one sane and compassionate, the other insanely violent and bent on destroying his counterpart, even if that act results in the destruction of both universes.