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: Jerome Bixby
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
Lieutenant Uhura: Nichelle Nichols
Ensign Chekov: Walter Koenig
Nurse Chapel: Majel Barret
Kang: Michael Ansara
Mara: Susan Howard

Beta XII – A. 
The landing party is taken prisoner by Klingons while a revolving cloud of light is seen keeping itself in the background. Kang, the Klingon captain, accuses Kirk of having killed his people and destroyed his ship in peace times and Kirk accuses Kang of having killed innocent people on the planet and Chekov is angry that they have killed his brother. Kang forces Kirk to beam all up so that he and the rest of his crew can take over the ship. 

Spock gets his captain's command right: Beam us up, all together (with a red light); he beams up the crew members only and later the Klingons who are met by security. He also wonders: The Klingons were far away at the time they had received the distress call. And Sulu wonders, too: Chekov never had a brother. Suddenly all phasers turn into swords or knives. Battles rage all over the ship between Klingons and the crew of the Enterprise and Spock finds a being which is different to both parties, pure energy. 

He also finds himself acting violently against Scott who had offended him. Kirk, Scott and Spock begin a fight and Kirk, who sees Spock fighting and feels hate in himself, suddenly realizes that something impossible happens. He stops them and all realize that something had manipulated them. Even a crew member needs a neckpinch after he had attacked Kirk. Mara, a Klingon, who is with them witnesses the developing of a plan: If they are all under the influence of the cloud, they will fight each other to provide hate, hate on which the cloud lives. 

Kirk goes into the machine room and tries to convince Kang, who immediately begins to fight. His hate makes the cloud appear and shine. All realize they are used by it to provide hate for it. Spock remarks that the end of fighting has made the being loose power, but to get rid of it for good, they need to find peace. They unite, refuse to be puppets of the being and laugh at it. The cloud vanishes.