When Kirk beams down with a large landing party to investigate planet Gamma Trianguli 6, a flower turns toward security guard Hendroff and sprays him with deadly spores. Scott reports that the matter-antimatter pods are inexplicably losing potency. He believes it to have something to do with the planet's unusual magnetic field. Kirk sends Valery and Marple to scout out the way to the nearest village. Spock then detects the presence of a humanoid watching them, and also detects planet-wide vibrations. Spock also finds a curious low density rock which explodes unexpectedly when he tosses it away. Spock runs in the way of another flower which is about to spray Kirk, and catches the darts in his own chest. McCoy injects Spock with masaform-D, but he does not respond, and Kirk asks Scotty to beam them all up. The Enterprise's matter-antimatter pods have already been drained by what Scott now identifies as some sort of beam from the planet, and Scotty's attempt to beam them up now fails, leaving the landing party stranded.
Spock recovers, but the landing party is then surprised by a rapidly moving electrical storm despite the absence of nearby clouds. The ground also begins to smoke. Valery attempts to report in by communicator, but his communication is jammed. The landing party goes to investigate, but Valery is tragically killed when he runs over one of the exploding rocks. The humanoid returns, and Kirk has Chekov and Spock create a diversion while he sneaks up on the watcher. Kirk attacks him, evoking tears. Kirk promises not to hurt him again, and the humanoid tells Kirk that his name is Akuta, chief of the primitive people he calls the feeders of Vaal. He appears to be in some kind of communication, since Spock notices antennae emerging from either side of his head. Kirk asks to be taken to Vaal, just as Scott reports that the Enterprise is being dragged into the planet by a tractor beam from the planet.
Akuta takes Kirk to Vaal, who appears to be a door into a hill in the shape of a dragon's head. Spock's tricorder shows that the entrance leads into the planet's interior, and is surrounded by a force field. When Akuta takes the landing party to meet with the rest of the people of Vaal, Kirk notices a strange lack of children, and finds that Vaal has forbidden love, providing ``replacements'' as they are needed. Seana introduces herself to Spock, and is greatly amused when he tells her his name. McCoy finds that the people are in perfect health without any disease or aging. Kirk and Spock then witness a ceremony in which the people of Vaal provide it with fuel. Chekov seduces Martha, and this is observed by two of the people of Vaal (Makora and his newfound girlfriend), who proceed to imitate it. This angers Vaal, who gives Akuta instructions.
As Kirk and party are resting, Akuta explains to his people that they must kill the strangers, and gives them instructions on how to bash their heads in using a heavy stick. They then disappear, and the Kirk and Spock go to confront Vaal. Vaal responds by calling a thunderstorm and striking Spock with a lightning bolt. The people of Vaal then attack, killing a security guard. As usual, the rest of the landing party fends off the attack and get off unscathed. Vaal weakens as the feeders are prevented from feeding him, and Kirk has Scott attack Vaal with the ship's phasers to weaken it further. This drains Vaal's power reserves, and frees the people from his grip. Spock accuses Kirk of giving the people the equivalent of the apple of knowledge and driving them from their Eden, but Kirk maintains that Spock's resemblance to the Devil is much more apparent than his own.