is tracing an unknown alien space probe that scanned the Federation, going back along its projected route. The probe self-destructed before it could be captured. As they start to lose the trace, they reduce speed and detect a vessel at extreme range, coming in on the same course as the probe. Kirk has the acting helmsman, Ensign Walking Bear, reduce speed. As it approaches, they determine it is twice the Enterprise
's size, made of crystalline ceramic, and surrounded by a unique energy field. The vessel attacks and the Enterprise
loses speed as they are surrounded by a globular force field, flexible enough to absorb any force.
The alien ship approaches them and opens fire through its force field and the Enterprise
is unable to return fire. The approaching ship proves the Enterprise
and its energy configuration changes into the appearance of a colored wing serpent. Ensign Walking Bear recognizes it as Kukulkan. The enemy ship stops firing and a voice says that he attacked them because he was angry, but one of them knows its name. He gives them one chance to succeed where their ancestors failed. Walking Bear, a Comanche, explains that the ship bears a resemblance to Kukulkan, a god of Mayan/Aztec legends. Spock confirms the information and the fact that similar legends are found in many ancient Earth cultures. Kirk concludes they're dealing with the space traveler that was the basis for all of the legends.
In Sickbay, McCoy disappears from in front of a patient. Scotty disappears from Engineering, and Walking Bear disappears on the bridge. A few seconds later, Kirk disappears as well. Spock takes command as the missing crew find themselves in a vast empty space aboard the alien ship. All they have between them is McCoy's medical kit. Walking Bear explains that Kukulkan left after promising to return one day. Kukulkan speaks and says he will show them the seeds of what he left behind and they must learn its purpose before he will appear to them. A vast city appears out of nowhere with a pyramid at its center.
As Spock and the Enterprise
crew work to locate Kirk and find an escape, Walking Bear examines the obelisks and confirms that the city is made up of architecture from various cultures. He says that the Mayans were told to build a city in accordance with the calendar that Kukulkan gave them. Once it was completed, he promised to return but he never did. They conclude that Kukulkan gave similar knowledge to other cultures but none of them built the correct city, and there must be a signaling device present. Kirk climbs to the top of the pyramid, which is surrounded by obelisks at each corner. he finds a platform with a crystalline insert. Below, Scotty and Walking Bear examine one of the obelisks, which has a serpent head at the top. As they turn it, Kirk realizes the sun is the key. They turn the head toward the platform and it generates a beam of sunlight. The men turn the other three obelisks and the combined beams generate a signal that causes Kukulkan to appear.
Kukulkan taunts them to use their "weapons of destruction" and wonders why they don't hate him. Kirk says they only fired in self-defense and Kukulkan insists he's their master. When they refuse to bow before him, he claims that they have forgotten him and it is his task to teach them. The city disappears to reveal an alien zoo and Kukulkan explains that all of the inhabitants lead a peaceful existence because they are mentally in their natural environment. Kirk insists that Kukulkan has no right to interfere in other cultures, and the alien says that its entire race was wiped out. As an experiment it visited Earth and tried to "civilize" it to Kukulkan's standards. When it was never summoned, it sent the probe and found warriors. Kukulkan boasts that its creatures, even a Capellan Power-Cat, lives peacefully despite its savage nature. McCoy notes the power-cat hates captivity and can release vast amounts of electricity when angered. Kukulkan says that they are but children but Kirk protests, noting that all children learn independence eventually. The alien refuses to listen further and attacks them for their "disobedience."
Aboard the Enterprise
, Spock determines that the force field can only respond to one force at a time. They use the tractor beams and warp engines in opposite directions and are thrown clear. As they return to Kukulkan's ship, the alien realizes something is wrong. He prepares to smash the Enterprise
and Kirk tries to find a distraction. They start pulling cables on the cages, freeing the power-cat. It goes berserk, shattering other cages. Outside, the Enterprise
opens fire on Kukulkan's ship, disabling the alien's power source. The power-cat corners Kukulkan and Kirk takes McCoy's hypo and injects it with a sedative. It throws Kirk clear with a power charge and then calms down.
Kirk demands the right to be heard and tells Kukulkan that they've been trying to live in peace. Since they have minds, they can't be lead by the nose. Kukulkan thinks of them as his children but Kirk insists that they’ve grown up and they don't need him any more. He says he will let them go their own way and returns them to their ship, then departs. McCoy wonders if the Vulcans had similar legends and admits that they were visited by alien beings that left much wiser. Kirk admits it's unfortunate but the price was too high.