The Enterprise runs into a being which once visited Earth, and influenced the Mayan culture.
Did You Know?
The plot point about Kukulkan actually being an alien lifeform was inspired by the Erich von Däniken book Chariots of the Gods?, especially claims like Indian sites in Peru having actually been created by aliens as methods of communication. See more
When the Enterprise is under attack, Uhura suddenly is coloured white in a wide shot. In the same shot, we see a lieutenant's stripes on Ensign Walking Bear's sleeves. See more
There's a line from Shakespeare...
Yes Bones, I remember it: How sharper than a serpents tooth it is to have a thankless child.
References Star Trek: Friday's Child