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 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, can even Scotty save Rimillia -- and the U.S.S. Enterprise? -- from total destruction.