 The Enterpise is approached by an ion-propelled craft to Scotty s liking; from it enters a female who renders everyone unconscious, then heads straight to Spock. When the crew wakes up, McCoy finds Spock alive in sick bay, his brain being surgically removed; Doc urges they must hurry to have any chance of putting it back in, if possible at all. Desperately Kirk gambles which planet the ion-trail they tried to follow actually leads to. It is populated by a race of large cavemen who attack; when the landing party captures one, he says they are small like  the Others  they fear as bringers of pain and delight. McCoy brings down Spock s body with a mechanical brain-substitute installed; they get in the Others  acclimatised dwellings and overpower similar female Luma by surprise, but she knows nothing, having a childish brain. Kirk s voice makes transmitter-contact but ignores where he is; an evil female overpowers the party, now captured and brought before a female panel, already wearing the controlling devices rendering all other males utterly obedient. They realize there is no superior intellect capable of handling the technology to maintain the subterranean complex, and apparently the  controller  they refer to is actually Spock s brain, which they must find and reunite with his body....