The episode begins with a Star Date of 5423.4. The U.S.S. Enterprise is sent to investigate a strange loss of contact with the colony of Gideon. Once they arrive, Captain Kirk, Commander Spock, and the rest of the crew find the colony deserted, save for a group of children who claim to be in charge. Led by a boy named Gorgan, the children display supernatural powers and remarkable intelligence levels. However, Kirk and Spock soon discover that the children have been manipulated by an alien entity known as "The Melkotians." The Melkotians had previously cast an illusion on the colonists, causing them to retreat in fear and leaving the children to fend for themselves. To save the colonists, Kirk and Spock must convince the children to reject the Melkotian's control and confront their fears.

The episode features performances from Leonard Nimoy as Spock, William Shatner as Kirk, and a young Kim Darby as Gorgan. The female characters are played by Barbara Baldavin as Mira Romaine and Mariette Hartley as Kara, two of the children who become essential allies to Kirk and Spock.

Ultimately, Kirk and Spock can convince the children that they must face their fears and reject the Melkotian's control. The colonists are successfully rescued, and the children are returned to their families. Through courage and determination, the crew of the Enterprise can overcome the Melkotian's illusions and save the colonists.