Chasing the stolen vessel Aurora, the U.S.S. Enterprise rescues the thieves just before the Aurora is destroyed. The group is led by Dr. Sevrin in a search for a mythological planet named Eden, a planet reputed to be a paradise.
Since one of Sevrin's group is the son of a Federation ambassador, Sevrin is held in protective custody while his followers are permitted to be free aboard ship. But when Spock is able to deduce Eden's location, Sevrin's followers free Sevrin and take over the starship.
Reaching Eden, Sevrin takes his people down to the surface in a shuttlecraft. When Kirk leads a landing party in pursuit, they find that the planet's vegetation all secretes deadly acid, and the fruits are all poisonous. Madly refusing to believe Kirk, Sevrin bites a piece of fruit and dies immediately. With his death, his followers are taken back aboard the U.S.S. Enterprise.