When a landing party beams down to the planet Gamma Trianguli VI, they find what appears to be an idyllic paradise. They quickly discover, however, that the planet is deadly, sporting plants that shoot thorns, rocks that explode, and incredibly accurate lightning bolts.
With a much reduced party, they encounter the planet's inhabitants. They are a peaceful, child-like people who call themselves the "Feeders of Vaal." They dress in flowers and bright paint, never reproducing because they don't age or die. Each day they "feed" Vaal offerings of food. Vaal seems to be a large serpent's head carved of rock, but is actually the terminal for an advanced, underground computer. Seeing Kirk and his party as a threat, Vaal takes them prisoner and tries to pull the U.S.S. Enterprise from orbit.
Kirk realizes that by depriving Vaal of the natives' daily offerings of food, the computer won't be able to convert the offerings into reaction mass. Thus weakened, Kirk uses the U.S.S. Enterprise's phasers to destroy Vaal. This leaves the natives on their own to discover birth, death, and the everyday ways of life.