Reger
Harry Townes

Marplon
Torin Thatcher

Hacom
Morgan Farley

Landru
Charles MaCauley

Crewman Lindstrom
Christopher Held

Tula
Brioni Farrell

First Lawgiver
Sid Haig

Tamar
Jon Lormer

Bilar
Ralph Maurer

Lt. Leslie
Eddie Paskey

Lt. O'Neil
Sean Morgan

While in orbit around the planet Beta III in system C-111 the crew of the Enterprise try to find some evidence to explain the disappearance of the starship 'Archon' over a hundred years previously. A landing party beams down to investigate further but on their return Sulu exhibits a strange, vacant expression and another crew member, O'Neil, is missing.

Kirk and Spock take another landing party down and, on arriving, are puzzled by the vacant looks on the inhabitants faces, similar to that on Sulu's. A clock strikes six and everybody around them suddenly goes berserk in a weird orgy of sex and violence. The landing party find shelter indoors where they meet three older men who enquire why they are not taking part in the 'festival'. Kirk replies that they are strangers visiting the area.

One of the men, Reger, offers them accommodation but another, called Halcom, disappears to inform the Lawgivers about the newcomers. Twelve hours later the activities of the 'festival' cease as suddenly as they had started and people act as if nothing had happened. Kirk queries what is happening, which makes Reger and the third man, Tamar, realise that the newcomers are 'not of the body'. At that moment two hooded Lawgivers arrive and order the Enterprise crewmembers to go with them. Kirk flatly refuses, causing the Lawgivers to pause while they commune to receive further advice.

This gives Reger and Kirk's people the opportunity to make their escape but they are pursued by a group of local inhabitants until Reger pulls them into an old building. He reveals that he is a member of an underground movement consisting of people who are immune to the mind control apparent in the others.

While he is talking a holographic image of a man-like being appears on the wall, identifying itself as 'Landru' and tells them that they will be absorbed into 'the body'. A sonic beam of some kind knocks Reger and the members of the landing party out and, when they awake, they find themselves in a prison cell. Shortly, they are reunited with McCoy and the previously missing O'Neil who are now exhibiting the same vacant expressions as the others.

An official called Marplen arrives with two Lawgivers who have come to take Kirk away for treatment. As they leave, Marplen reveals that he is a friend and returns two confiscated phasers to Spock. They overpower the Lawgivers and manage to find their way to Landru's 'audience chamber'. The holographic projection, which Spock and Kirk now realise is a computer-generated image, appears but weapons appear to be useless against it.

Kirk converses with it and ties it up in logical knots, finally convincing it that, by stifling creativity, it is destroying the 'perfect society' it was designed to oversee. The machine, unable to reconcile the paradox, destroys itself, releasing all the affected people from the mind control it had imposed and leaving the hooded Lawgivers helpless and confused.