While testing a new shielding device, the U.S.S. Enterprise is caught in the middle of a Klingon/Romulan battle. The Enterprise crew rescues a lifepod, and they are confronted by a Klingon who claims to know nothing of human existence. Convinced the Klingon is telling the truth, Captain Kirk hurries to Starfleet Headquarters in search of answers. But upon arriving on Earth, the Starship Enterprise crew finds that Earth is a vast jungle-like paradise where large, reptillian animals rule, with no signs of human life anywhere. Kirk must travel to the past in search of the key to the mystery, or face the destruction of the human race.