While testing a new shielding device, the USS Enterprise is caught in the middle of a Klingon/Romulan battle. When the Enterprise crew rescues a lifepod, 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, reptilian animals rule ... with no signs of human life anywhere. Now Kirk must travel to the past in search of the key to the mystery - or face the destruction of the human race.