Ever since Equality 7-2521 was a child, he has been more physically and mentally vigorous than his classmates, but his collectivist society has taught him that to be different is a sin. He tries unsuccessfully to suppress his curiosity, and for his crime of preference in desiring a position with the wise Scholars, the Council of Vocations assigns him a job as a Street Sweeper. For four years, Equality 7-2521 tries to acquit himself admirably in his vocation and to follow the rigid schedule of his life, but one day, he and International 4-8818 discover a secret tunnel from the Unmentionable Times before the Great Rebirth. International 4-8818 is too afraid to explore, but he is a loyal friend and does not betray Equality 7-2521, who begins to sneak away to conduct scientific experiments in the tunnel. Equality 7-2521 keeps a journal describing his experiences. This in turn forms the book's narrative. Significantly, Equality 7-2521 speaks always as "we" rather than "I", because his society does not have singular pronouns. He desires forgiveness for keeping the tunnel to himself and for thinking the thoughts of an individual rather than of the collective.
One day, as Equality 7-2521 is working in a Street Sweeper's brigade north of the city, he sees and is immediately attracted to the beautiful and proud Liberty 5-3000 among the Peasants. She also notices him, and they greet each other with subtle gestures. Eventually Equality 7-2521 finds an opportunity to speak with Liberty 5-3000 privately, knowing that International 4-8818 will not betray his contact with her. He has privately named her the Golden One, although such a unique name is a sin, and the two clumsily express preference for each other, despite their lack of a sufficient vocabulary to do so. Equality 7-2521 returns home happily, and upon being reprimanded for singing, he reflects on the fear and unhappiness of his coworkers while wondering about the Unmentionable Times and the Unspeakable Word, for which the Transgressor, a heroic man, had been executed at the stake.
After some time, Equality 7-2521 again speaks with the Golden One in the field. She has named him the Unconquered, although this act of individualization is forbidden, and she expresses a willingness to obey him. She brings him water to drink, and the contact between her hands and his lips spurs a new feeling of desire in Equality 7-2521. This confuses him, as he has previously only experienced sex through the indignities of the Palace of Mating.
During Equality 7-2521's private experiments in the tunnel, he rediscovers electricity, which is remarkable in a society that has lost all technology except for the candle, and he realizes that the Council of Scholars may not be as wise and omniscient as he had previously believed. He also concludes from the contents of the tunnel that the men of the Unmentionable Times had also known about electricity, and he decides to continue experimentation, ignoring his worries about breaking the law and finally succeeding in constructing a glass box that emits electric light. He begins to take pride in his own body and his accomplishments, and he decides that he will bring his invention to the upcoming World Council of Scholars, in the hope that it may help his fellow man, exonerate him for his crimes, and thereby allow him to rejoin society.
The day that Equality 7-2521 creates the glass box, he loses track of time and is caught for his absence and sent to the Palace of Corrective Detention for torture. However, he refuses to tell the Judges about his whereabouts in order to protect the light. The day before the meeting of the Council of Scholars, he escapes because the locks are rusted and the doors have no guards. (No one in their brainwashed society has ever thought to escape the Palace before.) He brings his creation to the Council, but rather than rewarding him for his ingenuity, they fear him and his creation and decide to destroy the glass box. To protect his creation, he takes it and escapes blindly through the window into the Uncharted Forest at the edge of the City, realizing in the process that he had actually built the box for himself and not to benefit society.
In spite of his initial despair at his sundering from his community, Equality 7-2521 thoroughly enjoys his first day of true freedom in the Uncharted Forest, reveling in his own strength and ability. In a stream, he sees his reflection for the first time, and he is delighted to see that the beauty of his body matches the worthiness of his mind. To his delight, the Golden One follows him into the Forest, despite her misgivings about being damned, and he teaches her not to suspect corruption in solitude as they have been taught. They begin traveling away from the city, sustaining themselves through the efforts of their bodies. Equality 7-2521 is joyful, and wonders how humanity's thoughts about morality have gone so wrong. At the same time, he and the Golden One wonder about the Unspeakable Word, without which they cannot truly express their love for each other.
Upon exiting the forest, Equality 7-2521 and the Golden One travel across mountains and find a house built in the Unspeakable Times, where they decide to make their home. They are surprised to find that the structure housed only two people, and the Golden One is fascinated by her reflection in the mirrors, while Equality 7-2521 wonders what his heart and nature are trying to tell him as he struggles through the vestiges of his old belief in collectivism. He reads the books in the house's library and discovers that the Unspeakable Word must be "I," representing the concept of the centrality of self that collectivism has removed from the culture.
Triumphantly, Equality 7-2521 explains in his journal that he needs no reason to exist except for himself, and that the error of collectivism was to assume that extreme altruism would free instead of enslave the individual. Only through egoism and the rejection of the worship of "We" can man achieve happiness and pride in his work, and friendship is earned rather than automatically forced on all men. Equality 7-2521 renames himself Prometheus and his wife Gaea, and he plans to raise his son as a free man and to return to the City briefly to gather like-minded men such as International 4-8818. He also decides to rebuild what the Great Rebirth destroyed from the Unmentionable Times, knowing that the power of the individual will ultimately achieve victory over the oppression and stagnation of the group.