Executive producer: Gene Roddenberry
Associate producers: Edward K. Milkis and Gregg Peters
Producer Fred Freiberger
Co- producer: Robert H. Justman
Story consultant: Arthur H. Singer

Writer: Edward J. Lakso
Dir.: Marvin Chomsky

Captain Kirk: William Shatner
Science Officer, First Officer, Cmdr. Spock: Leonard Nimoy
Dr. Leonard "Bones" Mc Coy: DeForest Kelly
Lt. Sulu: George Takei
Lieutenant Commander Montgomery Scott: James Doohan
Nurse Chapel: Majel Barret
Gorgan: Melvin Belli
Tommy Starnes: Craig Hundley
Mary Janowski: Pamely Ferdin

Stardate 5029.5

On Triacus the landing party finds the graves of the scientists, who were working on the planet. Only the children survive. They play happily and do not seem to care about what had happened to their parents. The crew of the Enterprise pays respect to the deads and tries to find out what happened. All except Spock experience a feel of fear which seems to increase the longer the stay on the planet. They beam back on the Enterprise, taking the children with them, along with the scientific mission's records to investigate.

On the Enterprise, the children show a strange behaviour. They stick together and reject any attempt of caring by the crewmembers. Dr. McCoy explains that that is a consequence of the shock they suffered, but the children begin to show weird powers, it becomes evident that there is more than shock aftermaths. Beaming the children on board they have unknowingly taken along Gorgan. Gorgan is an evil force, a being which misuses children for his own purposes of getting power over people. He lives on that power only. 

The children call him "Angel" and can make him appear as a semi-transparent humanoid form, by performing a sort of ritual chant. They do not recognize him as the destructive force which has killed their parents. He controls them and uses them to control the Enterprise crew. For this reason the children look at the crew members attending them with suspicion and use the powers that Gorgan gave them to help him in taking the Enterprise over. His intention is to be brought to other planet to exercise his power. And the children take over. 

They make Kirk unable to give orders and Scotty incapable of correcting the course. Other people on the Enterprise fall under the the children's power and mutiny. Anybody in their way is changed to some incapability. Kirk, sensing he has lost any control on his ship and crew, breaks down. When Spock sees his captain in total distress he takes him by his shoulders and only says: "Jim" – and Kirk becomes his old self again. 

Jim!

Once they have understood where the problem lays, Kirk and Spock know how to react. They overcome the mutiners and in front of the children  dismantle the shiny appearance of Gorgan and Spock shows them video tapes where they can see themselves cheerfully living with their parents and then the elders being attacked by an unseen force and die. The children realize that Gorgan was the cause of their death and look at him without giving him power. He ages, dissolves and ceases to exist.