A tropical planet's inhabitants are tended by a machine, Vaal, that regards the Enterprise as a threat.
Kirk leads a landing party to Gamma Trianguli VI, an idyllic-looking world where thorns, explosive rocks and lightning kill several security officers, while a force from the planet drains the Enterprise and slowly pulls it down with a tractor beam. The team meets Akuta, leader of the Feeders of Vaal, who receives commands from Vaal, a machine housed in a rock formation carved with a dragon-like head. Vaal keeps the villagers healthy and ageless but forbids affection and children, and the people feed it explosive minerals.
When Chekov and Yeoman Martha Landon kiss and a native couple imitates them, Vaal orders the strangers killed, and the landing party fights off the villagers. Kirk halts the feeding and has Scotty fire phasers at Vaal until it fails, freeing the ship and leaving Akuta's people to live on their own. Spock and McCoy debate the intervention, and Spock compares the outcome to the Garden of Eden story. Max Ehrlich wrote the episode.