Assignment: Earth

The Enterprise travels back in time to 1968 to observe a critical period in Earth's history. In orbit it intercepts a transporter beam from far across space, and Gary Seven appears on the pad, a human in a business suit with a cat named Isis. He claims to be on an urgent mission, but Kirk detains him. Seven escapes to a New York office equipped with advanced alien technology, where he reveals he was trained on another world by aliens who want humanity to survive. Helped and hindered by Roberta Lincoln, a secretary who believes he is a government agent, he sabotages the launch of an American orbital nuclear platform. Kirk and Spock cannot tell whether he is saving history or wrecking it. In the end Kirk trusts him, and Seven detonates the warhead harmlessly high above Earth, a warning against arming space. Spock hints that Seven and Roberta have an interesting future.