"That Which Survives" is the seventeenth episode of the third season of the American science fiction television series, Star Trek, first broadcast January 24, 1969, and repeated July 29, 1969. It was written by John Meredyth Lucas, based on a story by D.C. Fontana under the pseudonym Michael Richards and directed by Herb Wallerstein.

In the episode, the crew of the Enterprise visit an abandoned outpost guarded by a mysterious computer.

The Enterprise discovers a planet that appears young compared with the evolutionary stage of its surface life forms. Captain Kirk, Dr. McCoy, Lt. Sulu and geologist D'Amato, prepare to beam down to the planet to investigate. In the midst of dematerializing, they see a beautiful woman appear in the transporter room and try to stop the landing. She touches the baffled transporter technician, Ensign Wyatt, killing him instantly.

As soon as Kirk and the others materialize on the planet, the surface is rocked by a violent tremor. The Enterprise is also jolted as it is thrown 990.7 light years away from the planet by an unknown force. Dr. M'Benga examines Wyatt and learns his death was caused by complete cellular disruption.

The landing party loses communications with the Enterprise and they realize they are stranded. They split up and begin to scout around. As D'Amato surveys a rocky area, he comes face-to-face with the woman who appeared in the transporter room. Strangely, she says "I am for you" and knows his name. When she touches D'Amato, he too falls dead.

First Officer Spock orders the Enterprise back to the mysterious planet. En route, Chief Engineer Scott reports a vague feeling that the ship "isn't right" and orders Engineer Watkins to check equipment in a secluded area. The mysterious woman assassin appears again, kills Watkins and sabotages the Emergency Overload Bypass.

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