AllMovie

The Enterprise's attempt to evacuate a planet threatened with oncoming destruction backfires when Captain Kirk is left behind. This installment of the well-known science fiction series begins when Kirk, Mr. Spock, and Dr. McCoy arrive upon the planet, planning to warn the native tribal culture about a rapidly approaching asteroid. The trio are diverted from their mission, however, by the need to investigate an odd, monolith-like device that appears to be far beyond the native's technological ability. While exploring, Kirk enters the device and disappears; unable to find their captain, Spock and McCoy return to the Enterprise. Having given up on evacuating the planet, they now hope to redirect the asteroid away from the planet. Meanwhile, Kirk emerges from the device, having lost his memory. Unaware of his identity, Kirk joins the planet's native culture, and is welcomed as a new tribal leader. Meanwhile, however, Spock and McCoy are racing against time to save his life -- and that of everyone else on the planet.