A group of children is manipulated by an evil entity into taking over the ship.
The Enterprise answers a call from Triacus, where a group of scientists has died, and finds a group of children who are unaffected. The children are in the power of a being called Gorgan, who persuades them to take control of the ship and the crew with their mind powers and illusions.
The episode is often counted among the weaker ones, but it has a notable atmosphere and a creepy children's song. Kirk must break Gorgan's hold by forcing the children to confront the death of their parents. It remains a topic of discussion among fans for its odd tone.