The U.S.S. Constellation and its crew were destroyed by a giant robot ship which consumes planets for fuel, leaving only a guilt-ridden Commodore Decker aboard the crippled ship. Kirk beams over to begin repairs while Decker beams aboard the Enterprise. After Kirk loses radio contact with the Enterprise, the obsessed Commodore seizes command of the starship, determined to destroy the planet-killer, even at the cost of Kirk's ship and the entire crew.