The USS Enterprise, engaged in "historical research", travels back through time to 1968 Earth, where they encounter an interstellar agent planning to intervene in 20th-century events. Kirk and Spock are uncertain of his motives.

The agent, Gary Seven, is a mysterious man who claims to be from the future. He is accompanied by a black cat named Isis, who has psychic powers. Seven tells Kirk that he is on a mission to prevent a nuclear war from happening in the future. He recruits Roberta Lincoln, a young woman with a photographic memory, to help him.

Kirk and Spock are initially skeptical of Seven, but they soon realize that he is telling the truth. They agree to help him stop the nuclear war, and they travel to Washington, D.C. to prevent a group of scientists from launching a nuclear missile.

With the help of Roberta and Isis, Seven is able to stop the scientists from launching the missile. He then returns to the future, and Kirk and Spock return to their own time.

April Tatro played the role of the Cat Girl in the episode. She was a young woman who was possessed by a cat spirit. She was able to communicate with cats and control their minds. She helped Gary Seven and Roberta Lincoln to stop the nuclear war.

The episode "Assignment Earth" was originally written as a standalone half-hour television series. However, no network chose to order a pilot, so the script was reworked to fit into Star Trek as a backdoor pilot for the proposed Assignment: Earth series. The spin-off series was never produced, but the episode "Assignment Earth" remains a popular fan favorite.