Backtracking a mysterious alien probe, the U.S.S. Enterprise encounters a ship in the shape of a giant feathered serpent, which turns out to be Kukulkan, a god of ancient Mayan-Aztec legend. Its captain, Kukulkan, is actually a very long-lived benevolent entity who wants the humans to worship him just as the Mayans and Aztecs did. He insists on putting Kirk and others through a test to determine if they have grown sufficiently to receive his wisdom.