A mysterious being threatens to destroy the Enterprise if the crew is unable to solve an ancient puzzle.

William Shatner (Captain James T. Kirk)
Leonard Nimoy (Commander Spock)
DeForest Kelley (Dr. Leonard H. McCoy)

George Takei (Lt. Hikaru Sulu)
Nichelle Nichols (Lt. Uhura)
Majel Barrett (Nurse Christine Chapel)
James Doohan (Lt. Cmdr. Montgomery Scott)

• This episode won Star Trek: The Animated Series a Daytime Emmy Award for Best Children's Series. It was the first Emmy award for any Star Trek series.
  
• The title is from a line from Shakespeare's King Lear: How sharper than a serpent's tooth it is to have a thankless child."
  
• Ensign Dawson Walking Bear is voiced by James Doohan, and is the first Native American officer shown in Starfleet.
  
• This episode is very similar to the classic episode "Who Mourns for Adonais?", where an ancient Earth god is revealed to be an alien entitity. In this case it is the Mayan god Kukulkan.
  
• William Shatner pronounces Kukulkan differently from everyone else (as "kookla-kan").