The Enterprise is tracing the course of a probe that scanned Earth and sent a signal back along its course. They are attacked by a serpent-shaped energy field belonging to a feathered serpent who claims to be Kulkukan of Mayan-Aztec legend. It beams Kirk, McCoy, Scott, and Ensign Walking Bear aboard its ship to solve its puzzle before he will reveal himself. Aligning the mirrors in a strange city, they solve the puzzle and Kulkukan appears before them. It takes them to its ""zoo"" of violent creatures that now live in tranquility. Spock figures a way to break Kulkukan's force field and Kirk uses the diversion to free the animals. Kulkukan realizes that is not a god, but just a delusional old alien that once helped the younger races along. It leaves in sadness, leaving McCoy to remember the Shakespearean quote, ""How sharper than a serpent's tooth is an ungrateful child...""
Elenco: William Shatner , Leonard Nimoy , DeForest Kelley , George Takei , Nichelle Nichols , James Doohan , Majel Barrett , Majel Barrett .