Kukulkan was the last of an ancient serpent species who visited Earth's distant past. There he was instrumental in the technological and architectural advances of the Egyptian, Mayan, Aztec and South Asian civilizations. He was identified with the lore of Quetzalcoatl, as well as the legend of the Chinese dragon.
He also taught them new techniques in art and agriculture. He hoped that they would combine this knowledge to complete Kukulkan's city and live in peace; however, they did not. Had it been completed, it would have become an energy amplification system to be used as a signaling device to summon Kukulkan.
Kukulkan's visit to Earth was manifested in history by legends of a winged serpent creature that came from the sky bringing knowledge. These records were available in the library computer of the Constitution-class USS Enterprise.
Kukulkan (pronounced: Koo-kul-kon) was voiced by James Doohan.
In the German version of the episode, Kukulkan was spelled "Kulkulkan."
According to David Wise's audio commentary for this episode, "'One other thing you're gonna notice here is that we [Wise and co-writer Russell Bates] were at the recording sessions, everybody's recording sessions except McCoy's and Kirk's. Everybody else did their voice work in one session, and, unfortunately, because we were not at Bill Shatner's session, we were not there to correct his pronunciation of the lead guest character of this episode, so, undoubtedly you've noticed that everybody is calling him "Kukulkan" ( file info), which is the correct pronunciation, except for the captain of the Enterprise, who's saying "Kuklakan" ( file info), which Russell and I found very amusing. And we would refer to it as Kuklakan and Ollie, because when you see Kukulkan shortly he looks like a hand puppet, and he really looks like the hand puppet of the dragon Ollie from the old '50s TV kids' show Kukla, Fran and Ollie. So he is kind of Kuklakan and Ollie."
Despite the episode saying that "Quetzalcoatl" is another name for Kukulkan, an entirely different version of Quetzalcoatl appeared in the computer game Star Trek: 25th Anniversary.