Kirk, Spock and McCoy suddenly find themselves in an underground laboratory where they meet an attractive young woman who is not only mute but also an empath who can absorb someone else’s pain. When their captors make themselves known, they refuse to explain why the three men have been taken prisoner or why they and the young woman, whom McCoy has named Gem, are there. Inexplicably, they set about torturing them for no apparent reason. Fortunately, Gem’s empathic powers allow her to take away their pain, but only at great sacrifice to herself. When their captors tell Kirk that he must choose which of his men to die, their selflessness comes to the fore, leaving Dr. McCoy volunteering himself. They all soon learn that the object of the experiment is Gem herself.