"Turnabout Intruder" is a third season, as well as the final first-run, episode of the original science fiction television series, Star Trek. It is episode #79, production #79, written by Arthur H. Singer, based on a story by Gene Roddenberry, and directed by Herb Wallerstein. This was the last original episode of Star Trek to air on NBC.

Originally scheduled to air at 10pm on Friday, March 28, 1969, the network pre-empted it at the last minute with a special report on former U.S. President Dwight D. Eisenhower, who had died earlier that day. On June 3, 1969, after an absence of 2 months, Star Trek was brought back on a new night and time: Tuesdays at 7:30pm EDT. "Turnabout Intruder" was the only first-run episode to be shown in this new time slot.

In this episode, Captain Kirk becomes trapped in the body of a mentally unstable woman bent on killing him and taking over his command in his guise.

According to cultural theorist Cassandra Amesley, fans consider this to be one of the two worst episodes of the original series (the other being "Spock's Brain").

On stardate 5928.5, the Federation starship USS Enterprise rushes to answer a distress call from the planet Camus II, the site of an archaeological expedition that is exploring ancient ruins of a long vanished culture. Upon arrival, the landing party finds three survivors; among them are Dr. Janice Lester and Dr. Arthur Coleman. It is revealed that during their academy days, Dr. Lester and Captain Kirk were intimately involved; however, Lester's inability to achieve command of a starship, which she believes had to do with her gender, caused the two to drift apart.

Medical scans reveal that the survivors are suffering from radiation exposure which threatens their lives. Dr. Coleman, Science Officer Spock and Chief Medical Officer Dr. McCoy go off to tend to the other survivor, leaving Lester and Kirk alone for a moment to talk. Kirk finds Lester has been driven to the point of madness by her Self hatred personality complex and complains about the agony of being a woman in a male-dominated existence. She traps Kirk in an alien device which can swap the personalities between two individuals. Joining Kirk in the machine, Lester's essence is transferred into Kirk's body while Kirk's mind enters hers. Now functioning as Kirk, Lester-in-Kirk tries to strangle her former body, inside which Kirk is trapped.

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