The Enterprise is investigating a planet that appears to be too young to account for its level of evolution. As Kirk dematerialises while beaming down to the surface with a survey team consisting of McCoy, Sulu and D'Amato he sees a mysterious woman appear in the transporter room. She touches the transport operator, Ensign Wyatt, who dies immediately. Arriving on the planet's surface the survey team is shaken by a large ground tremor. At the same time, the Enterprise disappears from orbit, having been thrown 990.7 light years away from the planet by an unidentified force, leaving Kirk and the others stranded.

Wyatt's body is examined by Doctor M'Benga who finds that the crewman died due to complete cellular disruption. The survey team starts to explore their surroundings and split up. D'Amato, a geologist, is confronted by a mysterious woman who seems to appear from nowhere and addresses him by name. He allows her to approach him and when she touches him he dies in the same way as Wyatt. On board the Enterprise Spock assesses the ship's location and charts a course back to the planet. When the ship goes into warp they discover that the strange force which transported them away from the planet has reassembled the ship slightly out of phase with normal space.

Their speed continues to increase uncontrollably and Scotty is sent on a highly dangerous task to cut the magnetic field which regulates the flow of fuel to the matter / antimatter chamber. By the time he accomplishes this, the ship's speed has increased to warp 14. On the planet the remaining survey team members bury D'Amato's body and make camp, taking turns at keeping watch as night begins to fall. After Sulu is approached and injured by the touch of the mysterious woman Kirk has a hunch that she can only 'focus' on one person at a time. When she reappears calling Kirk's name Sulu and McCoy stake their lives on Kirk's idea being correct and place themselves between him and the woman, successfully shielding him despite her entreaties and attempts to touch him.

The three Enterprise men manage to survive the night and explore the area further to try and find out where the mystery woman is coming from. They discover a camouflaged entrance hidden in a rock face and go in, finding themselves in a room with a computer system and a large viewscreen. The computer is active and creates three identical images of the mystery woman, one for each of them, which it will be impossible to defend against indefinitely. The Enterprise arrives back at the planet and resumes a standard orbit. Spock manages to locate Kirk and the others with the ship's sensors and beams down to their location with a security man. They arrive in time to rescue the survivors, disabling the part of the computer that is creating the image projections and causing the 'women' to disappear.

The woman's face appears on the computer's viewscreen and delivers a pre-recorded message in which she identifies herself as Losira, 'commander of this outpost'. The recording tells how her people, the Kalandans, were infected by a deadly organism created during their terraforming of the planet. She was the last survivor and the computer used her image in its attempt to protect the outpost until help arrived. This was never to come as their supply ships had spread the organism throughout the entire Kalandan race that has long since been extinct.