Lt. Kyle
John Winston

Akuta
Keith Andes

Sayana
Shari Nims

Makora
David Soul

Hendorff
Mal Friedman

Yeoman Martha Landon
Celeste Yarnall

Marple
Jerry Daniels

Ensign Mallory
Jay Jones

Kaplan
Dick Dial

A landing party beams down from the Enterprise to examine the unexplored planet Gamma Trianguli VI. Within minutes of arriving in what appears to be a paradise, a crewman called Hendorff is killed by a plant which fires poisonous darts into his chest and the rocks on the surface are found to be unstable and explosive. Also, something on the planet is causing a drain on the Enterprise's antimatter pods, reducing their power to the point where the landing party cannot be beamed back aboard.

Two more of the landing party are killed before one of the planet's natives is seen spying on them from the undergrowth and is captured for questioning. He tells them that his name is Akuta and that he is the leader of the People of Vaal. In the meantime, to add to their problems, a mysterious tractor beam has locked on to the Enterprise, pulling it towards the planet and certain destruction.

Akuta leads Kirk and the remaining members of the landing party to Vaal, an entity which apparently lives in a cave whose entrance is surrounded by an enormous snake's head carved in the rock. Spock's tricorder reveals that this is the source of the power drain but they cannot get near it as it is protected by a force field. In the native village McCoy discovers that there is no reproduction and that the people do not seem to age.

Periodically they are called to the cave to worship the snake's head Vaal, where Kirk and Spock observe a ritual 'feeding', which leads Spock to surmise that this amounts to a kind of 'refuelling'. After convening with the 'god', Akuta tells the villagers that Vaal has ordered the strangers to be killed. The landing party is attacked but the natives are unused to physical combat and are soon overpowered and locked in their huts.

Unable to call the natives to him for worship, Vaal's power starts to diminish from lack of food. The Enterprise fires phasers at the cave from orbit, eventually destroying its force field and rendering it powerless and effectively dead. Without the control of Vaal the people of Gamma Trianguli VI can start to resume a more normal evolutionary existence, enjoying love and growth for the first time they can remember.