Unlike most planets, Rimillia does not spin upon its axis, so its day and night sides are subjected to perpetual extremes of hot and cold. Habitation has only been possible on a thin band of the planet's surface, until now. Using gigantic impulse engines of unimaginable power, the alien Dumada intend to start Rimillia rotating, rendering the entire world fit for colonization. Yet some fear the enormous stresses involved may instead tear the planet apart. Assigned to assist the Dumada, Captain Kirk must rescue a kidnapped scientist vital to the rotation project. But, once the giant engines are activated, Scotty must save Rimillia, and the U.S.S. Enterprise from total destruction.