As the Enterprise approaches planet Omega IV, they find another starship, the U.S.S. Exeter, in orbit. Kirk, Spock and McCoy beam aboard to find the ship abandoned but strewn with uniforms and crystals. The last log entry from the ship's surgeon tells them they have been infected with a deadly virus brought aboard from a returning landing party. Kirk's party beams down to the planet's surface and finds there is one Exeter survivor: Captain Ron Tracey. He has apparently ignored the Prime Directive and has taken sides in a local dispute supporting the Kohms against their arch-rivals, the Yangs. As McCoy tries to find a cure for the virus, Spock and Kirk try to make sense of the situation. They eventually realize there is an odd parallel with Earth's own history.