An energy being that feeds on hatred sets the Klingons and the crew against each other.
The Enterprise meets a Klingon ship that has been devastated, and its commander, Kang, blames the Federation. Weapons are replaced by swords, and both crews seem to hate each other and fight more fiercely. Kirk and Kang realise that an energy being is feeding on their anger and driving them to kill.
The story is a classic of anti-war storytelling with two enemies united against a common threat. It features Michael Ansara as Kang, who returned in later series. The closing laughter of both crews at the end is fondly remembered, and the episode is regarded as one of the best of the third season.