As the U.S.S. Enterprise nears the planet Pollux IV, a huge, green hand made of energy materializes in space, catching the U.S.S. Enterprise and holding it captive. Kirk and a landing party are transported to the planet's surface where they find a being who claims to be Apollo, the last of the Greek gods who dwelled on ancient Earth's Mount Olympus. All the other gods, Apollo tells Kirk, died of loneliness when they left their home of Mount Olympus. Apollo's plan for the U.S.S. Enterprise crew is that they settle on Pollux IV and worship their god, Apollo.
Sensors show that Apollo's god-like abilities come from an organic ability to use energy from sources outside himself. His apparent powers include storms, thunderbolts and an ability to grow into a giant, towering above the Enterprise landing party.
Attempts to foil Apollo's plans are hampered by Lieutenant Carolyn Palamas, who has fallen in love with the god and whom Apollo decides will be his bride. Scotty, who has beamed down with the party, had harbored hopes of romance with the young lieutenant and objects to Apollo's attentions. Apollo's response is to hurtle the engineer through the air by way of a thunderbolt.
Spock determines that the god's powers come from his temple. When Kirk tells Lt. Palamas to reject Apollo, which she reluctantly does, the captain uses the U.S.S. Enterprise's weapons to destroy Apollo's temple. Apollo, rejected by a mortal woman and bereft of his powers, spreads himself upon the winds to join his fellow gods.