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The Enterprise tries to save an unidentified ship plunging into a supernova but is dragged into a universe where time runs backwards. Their only hope to return home is Commodore Robert April, the first commander of the Enterprise, who is onboard on a retirement trip.
The Enterprise crew encounters an alien who visited Earth in ancient times and became known as the Mayan god Kukulkan.
After passing through an unusual energy field, the Enterprise computer develops a playful--and potentially lethal--personality.