Down on the planet of Beta III, Sulu and a Red Shirt are being hounded by a group of brown-cloaked figures wielding the world's longest Swiss Roll; the Red Shirt runs off and disappears, while Sulu is beamed aboard the Enterprise, but not before being touched by the Swiss Roll, converting him into a hippie ditz. Concerned by this turn of events, Kirk takes a landing party down to scope things out.
What they find isn't promising; a giant Lotus-Eater Machine disguised as a 19th-century town, where every Victorian-clad citizen is nice and friendly and not at all a Stepford Smiler...until the evening falls, during which they, well...aren't. Nice, friendly, and a Stepford Smiler, I mean; for the entire night, everyone goes crazy and destructive, like the entire cast of Equilibrium went off their Prozium at the same time and started going through the withdrawl symptom of violent mood swings. The away team finds shelter at a local boarding house run by a man named Reger, who grows interested in their lack of going nuts like the rest of the town and questions if they're "Archons", referring to the crew of the ship that the Enterprise came to the planet to find; Kirk refuses to say, thanks to that pesky Prime Directive.
The next day, though, someone tips the Brown Cloaks off to the landing party, and attempts to "absorb" them into the Lotus-Eater Machine. Fortunately, Kirk's uncanny ability to Logic Bomb any computer-like being saves the day, and they escape with Reger to a safer location...but not without picking up the Red Shirt, now a member of the machine, against Reger's warnings not to. Once out of danger, Reger explains the whole thing: he is a resistance fighter against the Brown Cloaks and their master, Landru, who controls the people via Mind Control, and polices them with the Brown Cloaks; anyone out of its thrall is told You Will Be Assimilated (so, nothing like the Borg, then), and are killed if they can't be. Even worse, Landru has the ability to pull entire starships out of the sky to assimilate its crew, which is what it's doing to the Enterprise, currently (though with heat beams...yeah, major Artistic License Physics, there). Unfortunately, since the Red Shirt has been assimilated, Landru finds the group through him, and knocks them out in an attempt to capture and assimilate them.
The away team finds themselves deep in Landru's sanctuary, on call to be assimilated, and with no hope of escape now that the Brown Cloaks have adapted to Kirk's Logic Bombs (again, no Borg similarities here). In a stroke of luck, though, it turns out Marplon, the master assimilator, is a member of Reger's underground, and manages to keep Kirk and Spock from being absorbed. After subduing their captors (why they didn't do that, before, is anyone's guess, as is why Landru bothered to bring them to his inner sanctum to assimilate them instead of having the Brown Cloaks touch them with their Swiss Rolls), Kirk has Reger and Marplon take them to Landru...which turns out to be a giant computer (what else?).
As it turns out, Landru was once a real person, a leader of the colony on the planet, who built the machine to help him keep the peace over the people; once Landru died, the computer took over his name, identity, and purpose, and went through a Zeroth Law Rebellion, force-assimilating people into the Hive Mind in order to keep order. When the Archon crew came, it saw them as a threat to its perfect society, and assimilated them, just like it's trying to assimilate the crew of the Enterprise. Unfortunately for it, Kirk regains his Logic Bomb mojo and makes it explode by forcing it to realize it's harming the very people it sought to protect by stifling their free will and creativity (how Anvilicious of you). Having ruined yet another idyllic society, Kirk leaves the ship's sociologist to guide the colony to a more "human" existence, and goes gallivanting off to a new adventure.
This episode contains the following tropes:
- Absurdly Dedicated Worker: Landru continues to follow its original programming without seeing the damage it is causing the society.
- A.I. Is a Crapshoot: Landru. At least it had good intentions.
- Alien Non-Interference Clause: This is the first episode of the series to mention the Prime Directive. Kirk decides that it doesn't apply in this case because it only applies to "living and growing" cultures, while this one is stagnant.
- Breaking Speech: Kirk's Logic Bomb on Landru causes the computer to self-destruct.
- Frequently-Broken Unbreakable Vow: Never interfere...unless an ancient computer has restricted population to only two modes of behavior: Mindless Stepford smiler and Brazilian soccer fan.
- Funny Background Event / No-Sell: Done by mistake in this case. At one point the Enterprise crew members down on the planet dodge some collapsing rubble from a building. In the background of the scene, one huge chunk of debris hits one of the extras on the head and yet the guy is completely unaffected. Either nobody noticed it, or else they didn't have the time or possibly budget to re-shoot the scene.
- La Résistance: Reger and his buddies are this, but because they're so scattered and lacking in numbers/influence, aren't able to do much.
- Logic Bomb: Kirk, as was to be expected, does this to the computer.
- The Mole: Marplon, the assimilation overseer who turns out to be a member of the resistance, presumably placed there to identify and if possible assist new resisters.
- Orbital Mechanics Does Not Work That Way: "Heat beams" (known today as "lasers") are not going to threaten a starship's orbit. Especially to the degree to which irreversible orbital decay will occur in six hours.
- Plot Hole:
- What is the point of the violent "Festival"? The novelization offers the explanation that it's how the computer keeps the population from growing. Or likely release the emotions that are otherwise kept under wraps for the rest of the year; have to be vented sometime, similar to Vulcans.
- Why doesn't Landru just have the Lawgivers zap the whole crew immediately, as they did Sulu in the teaser, rather than take them one by one to the absorption chamber?
- Red Shirts: Subverted. Everybody lives! Maybe because they weren't actually wearing red shirts?
- Simpleton Voice: Bilar, the first to address the landing party seems to be this, which he seems to do in a childlike voice which was to suggest that he was mentally slow. It may not have been fully attributed to being "part of the body" as other adults appeared to speak normally. When they inquire about lodging, he introduces them to Tula, whose father (Reger) owns a hotel.
- Lee Mailer who played Bilar was told that the colony was something like "late 19th century New England," so he affected an old New England accent: "A-yah, come for festival, a-yah-a?" But nobody else got the message, so he was the only one with the accent.Bilar: Your daddy can put them up, can't he?
- Lee Mailer who played Bilar was told that the colony was something like "late 19th century New England," so he affected an old New England accent: "A-yah, come for festival, a-yah-a?" But nobody else got the message, so he was the only one with the accent.
- Utopia Justifies the Means: Landru becomes an absolute dictator, deprives its subjects of free will, and subjects them to the Red Hour festival, out of a genuine desire to help the people by creating a society without sickness, war, or conflict. It is trying to follow its programming, and destroys itself as soon as it calculates (with Kirk's help) that it is damaging the people it is supposed to protect.
- You Will Be Assimilated: Both Sulu and McCoy get temporarily brainwashed by Landru.