 The Enterprise answers a distress call, but the landing party is told to surrender by Rojan, who establishes mind control over them, as Hanar, also of his kind, the Kelvins, which has assumed human form posing as colonists, does aboard the starship -entering and leaving on his own-, declares the Kelvins end humanity s former way of existence, and improves the ships engines so in  merely  300 years they can make an intergalactic voyage to the Andromeda system. When a Vulcan mind probe to attempt escape fails, as punishment two crewmen are instantly reduced to solid organic matter, one crushed irrevocably but the other restored as a demonstration. Spock deduces such is the Kelvins  normal state. Kirk tells him to fake sickness and modify diagnostic equipment to attempt reversing the Kelvin nervous control field; McCoy is allowed to bring him aboard the Enterprise. Spock finds the engine modification is cased in an impenetrable material and proposes a sabotage which will make the ship explode while it passes a natural energy barrier but Kirk fails to order the explosion; the Kelvins were aware of the plan and  neutralize  all non-essential human crew as revivable matter. Spock observes the Kelvins have sacrificed all non-intellectual mind functions, and are thus vulnerable in their present human form to sense-stimulation, so the remaining crew bombards them with sensuality, which indeed throws them off balance, even striving amongst each-other. When they realize being weakened, Kirk makes them an an offer...