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After passing through an unusual energy field, the Enterprise computer develops a playful--and potentially lethal--personality.
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.