AllMovie

Investigating an ancient civilization, the crew of the Enterprise discovers the preserved brains of three of the civilization's final survivors. The alien beings propose a temporary exchange, wherein they would inhabit the human bodies long enough to construct robotic shells for their minds. After deliberation, Captain Kirk (William Shatner), Mr. Spock (Leonard Nimoy), and Dr. Ann Mulhall (Diana Muldaur) volunteer for the procedure. The change is successful, but proves to have unexpected side effects -- including an acceleration of bodily processes that means the task must be completed within a brief period of time. Complicating matters further is the fact that the being inhabiting Spock's body no longer wishes to cooperate with the original plan.