Kirk beams down with Spock, McCoy, sociologist Lindstrom and two security guards. They are greeted by a man walking past them with his hand on his heart and a broad smile on his face. Spock notes his mindlessness and vacant contentment, to which Kirk replies, 'Like Sulu.'

They are greeted by another man (later shown to be Bilar) who, realising they are strangers, asks if they have come for the Festival and introduces them to Tula, the daughter of Reger who runs a boarding house while others walk past them slowly and grinningly. Tula asks if they are from the Valley which Kirk, jumping at the opportunity to explain their ignorance about local customs and norms without revealing they are aliens, answers in the affirmative.

The tower clock strikes six, ringing in the Festival which turns out to be a (presumably annual) period of ritualised permissiveness during which a severely restrictive society temporarily encourages individual expression and the breakdown of norms and hierarchies, functioning as an emotional safety valve for an oppressed population, similar to Carnival, Mardi Gras or the Holi festival in India. At the sixth stroke the content populace turns into a violent mob: people scream, houses are vandalised and looted, stones thrown, fistfights initiated and women chased and assaulted. This scene, besides others (i.e. claiming to be peaceful while killing all who continue to resist or claiming to live without fear which is the very fabric of their society), perfectly demonstrates how ingrained cognitive dissonance is in collective identities.

The landing party storms into Reger's house where they meet him and two other men, Tamar and Hacom, who discuss their behaviour. Hacom insists they should be at the Festival while Tamar points out that, while acknowledging that Landru is everywhere, the Valley may have different ways.
Meanwhile Lindstrom tells Reger that his daughter shouldn't be out there, to which he replies, 'It's Festival. It's the will of Landru.'
After accusing Tamar of mocking the Lawgivers and the strangers of not being of the Body, Hacom leaves in a huff.

Reger shows the landing party to their quarters and is surprised when Kirk tells him he has no intention to attend the Festival, but that he wants to know more about the Festival and Landru. Reger doesn't finish a question and walks out as Kirk watches the rioting in the street.
As the tower clock strikes six again in the morning, the rioters freeze and then start walking about as if nothing had happened.
A distressed Tula, presumably having experienced a sexual assault, comes home and cries on her father's shoulder who reassures her, 'It's all over now.'
Confronted by Lindstrom again about not having kept her in the house, he repeats, 'It's Landru's will.'
Kirk once again asks about Landru and, after admitting to not having attended the Festival, Reger concludes that they are not of the Body.

In the drawing room Reger, Tamar and the landing party are gathered around the sedated Tula. Reger eventually asks the question of whether they are Archons, which Kirk evades by asking, 'What if we are?'
Tamar suggests hiding them from the Lawgivers lest Landru should find out. Suddenly the door bursts open, and Hacom returns with two Lawgivers, accusing Tamar of having mocked them and the others of having scorned the Festival.
One of the Lawgiver executes Tamar with his staff; when asked for the reason by Kirk, he replies, 'Out of order.'
He then tells the others, 'You attacked the Body. You have heard the Word and disobeyed. You will be absorbed.'
He leaves the question as to what he means unanswered and orders them to follow them.
Kirk refuses, and the perplexed Lawgivers turn towards each other.
Spock notes that they aren't prepared to deal with outright disobedience and asks Kirk how he knew. Kirk replies that all circumstances indicate a 'compulsive involuntary stimulus to action', which Spock considers 'logical'.
Assuming they simply hadn't understood the order, the Lawgiver rephrases it and tells them to accompany them to the absorption chamber, to which Kirk replies that they will talk to Landru in their own time.
Meanwhile Spock inspects the staff of one of them just to find that it's merely a hollow tube without any mechanism.
Reger offers to bring the others to a safe place, and they walk out between the frozen Lawgivers.

On the street where the men are masking (i.e. copying the behaviours and mannerisms of those around them to avoid drawing attention) by walking in slow motion and foolishly grinning, Lindstrom spots the man who assaulted Tula and points him out to Reger, shouting, 'That's the man', to which he replies, 'No, it wasn't Bilar. It was Landru.' His response highlights how personal responsibility in collective identities is deflected to authority ('just following orders') or the group as a whole ('everybody does it').
Suddenly everybody around them stops in their tracks, turns towards them, picks up anything that could serve as a weapon and approaches them. Reger explains that Landru is summoning the Body, and Spock concludes that it is telepathy.
The landing party defend themselves by stunning the people who get too close while retreating. One of them turns out to be O'Neil. Kirk points out that he's one of their men, to which Reger replies, 'Not anymore. He's been absorbed.'
It turns out that absorption, carried out by the touch of a staff or, more formally, in an absorption chamber, is the mechanism of introducing newcomers to the narrow norms of the Body as well as getting members who deviate from them back in line, corresponding to the earthly processes of social conditioning, social sanctioning and behavioural 'therapies'.
Kirk orders his men to bring him along, anyway, against Reger's protests that, once O'Neil wakes up, Landru will find them through him.

They enter a secret chamber where Spock detects strong power generations radiating in all directions.
Reger tells them that when the Archons came, they were free and opposed the will of Landru, and therefore many were killed and many more absorbed. 'And if the others come...' - 'What others?' Kirk enquires, to which Reger replies, 'Those like you and me, who resist Landru.'
He explains that resisters are organised in groups of threes, with only one known to each other, and that the late Tamar had been his contact.
He then claims that the Archons had been pulled down from the skies by Landru, and Spock confirms that this could have been technically possible.
Kirk contacts the bridge, and Scott tells him that the Enterprise is under attack from heat beams generating from the planet's surface.
Spock detects sensor beams, upon which Reger murmurs, 'Landru', folds his hands and lowers his head.
A projection of Landru appears and accuses them of being destroyers and bringing infection. He claims, 'You have come to a world without hate, without fear, without conflict. No war, no disease, no crime. None of the ancient evils. Landru seeks tranquillity. Peace for all. The universal good.' (It should be noted that society on Beta III is only harmonious because, unlike on Earth, no outgroups exist.)
He continues, 'You will be absorbed. Your individuality will merge into the unity of the good, and in your submergence into the common being of the Body, you will find contentment and fulfilment. You will experience the absolute good.'
Throughout the appearance Reger couldn't bring himself to look at Landru.
As his projection fades out, the party are attacked with a sonic weapon that knocks them out.

They wake up in the anteroom of the absorption chamber. Kirk considers confusing the Lawgivers into inaction again, but Spock points out that such a serious error wouldn't go uncorrected.
McCoy and another crew member return to the anteroom after having been absorbed. McCoy appears blissfully vacant and doesn't seem to know Kirk (whom he addresses as 'friend') who asks him, 'Don't you know me?' to which he replies, 'We all know one another in Landru.'
A Lawgiver orders Kirk to come with him. Kirk refuses, to which he replies, 'Then you will die.'
He eventually follows to the absorption chamber where he is strapped to the wall while another Lawgiver, operating the control booth, is relieved by a man called Marplon. Afterwards Spock is called and follows to the chamber where a vacant-looking Kirk on his way out greets him in the local manner. Spock frowns, possibly considering that he was putting on an act.
Marplon reveals himself to be the third resister in Reger's group (whom he deems immune to absorption) and welcomes what he considers the return of the Archons. Back in the anteroom Spock, Kirk and Lindstrom start discussing the next steps, arousing McCoy's suspicion. They manage to appease him with a multitude of Landruian platitudes to which he replies, 'Blessed be the Body.'
They move to a remote corner where Spock describes the society as soulless: 'The peace of the factory, the tranquillity of the machine. All parts working in unison.'
Kirk and Spock discuss how unexpected events disrupt their routine until new orders are received and conclude that these orders come from Landru, and that Landru is not a human being.
Kirk who, on multiple occasions throughout the series, violates the Starfleet's Prime Directive of non-interference when he considers it ethically necessary, suggests pulling the plug on Landru. When Spock reminds him of it, he claims that the directive only refers to 'living, growing cultures'.
They are joined by Reger and Marplon whom they grill about Landru, stressing that they must know about him to free them.
McCoy approaches the group and screams that they are not of the Body, calling them traitors. This time Kirk's platitudes fail to work, and McCoy attacks and chokes him but is eventually defeated.
As the two Lawgivers enter, Kirk and Spock hide behind the pillars of an arch through which they pass, knock them out and take their robes.
Kirk presses Marplon and Reger, who look increasingly terrified, about the location of Landru.
Spock checks the situation on the Enterprise and is told by Scott that it got worse while Kirk orders him to put a guard on Sulu.
Asked again about Landru, Reger replies, 'There was war. Convulsions. The world was destroying itself. Landru was our leader. He saw the truth. He changed the world. He took us back, back to a simple time. A time of peace and tranquillity.'
Marplon reassures Kirk that Landru still exists. 'He's still alive. He is here now. He sees. He hears... We have destroyed ourselves!'
After Kirk points out that freedom has to be earned and suggests finding Landru, Reger panics at the thought of facing the tyrant; he shouts out that he was wrong, submits to Landru and calls the Lawgivers before being silenced by the Vulcan Neck Pinch, demonstrating that even those on the individual end of the neurological spectrum can break under increased pressure.
Still hesitant, Marplon brings Kirk and Spock to the Hall of Audiences where Kirk demands to see Landru and claims that his people are Archons.
Landru's projection appears and accuses him of having invaded the Body, causing great harm, and that he and his men, as well as anybody who saw them or knew of their presence, will have to be obliterated to cleanse the memory of the Body.
Kirk and Spot aim their phasers at the wall behind the projection and blast a hole into it, finding out that Landru is nothing but a giant computer, programmed thousands of years ago by a man of that name and illustrating that collective identities are mere constructs existing only in the minds of their followers.
Landru claims that the good of the Body is the prime directive, and Kirk argues that he has violated his own directive: 'The Body is dying.'
Kirk asks him, 'What have you done to do justice to the full potential of every individual of the Body?' to which Landru replies, 'Insufficient data.'
Kirk continues, 'Without freedom of choice, there is no creativity. Without creativity, there is no life. The body dies. The fault is yours.'
Two Lawgivers come in and ask Landru for guidance.
Kirk insists that, as per his own prime directive, Landru must destroy himself for being evil.
After a few more attempts at philosophical musings, the computer dies.

In the end Sulu and, presumably, McCoy are back to normal, the Enterprise is safe, and Lindstrom and some experts remain on Beta III to help develop a healthy society.

The difference between Beta III and Earth is that no Archons will come to our rescue, so the entire burden of saving society rests on the shoulders of our own resisters.