Set in the 24th century, Fahrenheit 451 tells the story of the protagonist, Guy Montag. At first, Montag takes pleasure in his profession as a fireman, burning illegally owned books and the homes of their owners. However, Montag soon begins to question the value of his profession and, in turn, his life. Throughout the novel Montag struggles with his existence, eventually fleeing his oppressive, censored society and joining an underground network of intellectuals. With his newfound friends, Montag witnesses the atomic destruction of his former city and dedicates himself to rebuilding a literate and cultural society.
At the beginning of the novel, Montag develops a friendship with his 17-year-old neighbor, Clarisse McClellan, whose humanistic outlook and inquisitive nature prompt Montag to examine his life. Soon, he realizes he is unhappy and no longer loves his wife, Millie. Meanwhile, Millie is unwilling to deal with reality and instead chooses to immerse herself in interactive television, seashell radio, and an addiction to tranquilizers. Unfulfilled by his occupation and discontent with a society unconcerned with reports of an impending atomic war, Montag begins to question the ways of the world. Specifically, Montag wonders why books are perceived to be so dangerous and why some people are so loyal to them. What power lies in books?
Driven by his increasing uneasiness, Montag steals a book from a collection he is sent to burn. At the scene of the burning, Montag is shaken when the owner of the books, an older woman, refuses to leave her home. Instead, the woman sets fire to her kerosene soaked house and remains there as it, and she, are destroyed by flames. The woman's dedication to her books makes Montag realize that perhaps the happiness he lacks can be found in books. After the burning, Montag returns home, feeling ill as he relives the woman's horrific death. He begins to realize that although, over the past ten years, he thought he was serving society as a fireman, he was actually purely an instrument of destruction. That night, in a discussion with Millie, Montag learns that his friend Clarisse was killed by a speeding car more than a week earlier. Upon hearing this news, Montag feels even more ill. He falls asleep that night with his stolen book hidden underneath his pillow.
The next day, Montag refuses to attend work, claiming that he is sick. His boss, Captain Beatty, visits Montag that morning, and appears to somehow be aware of the internal struggle Montag is suffering through, and that Montag might possess books. Beatty lectures Montag about the offensiveness of books and the superiority of their current society, where homogeneity and structure are mandated, to the old society where free thought was encouraged and people were permitted to express differing opinions. During Beatty's visit, Millie nervously organizes the bedroom and tries to pull Montag's pillow away. When he won't let her, she puts her hand underneath it and finds the hidden book. Millie is astonished, and although she does not directly give up her husband, she asks Beatty what would happen if a fireman brought a book home. Beatty says firemen are allowed to bring a book home, but must burn it within 24 hours.
When Beatty departs, Montag retrieves some 20 books that he has stolen from alarms over the years and begins to read. Unsure as to what to do next, Montag recalls meeting a retired professor, Faber, a year earlier and discussing with the old man the value of ideas. He decides to visit Faber, who is at first afraid to speak with him, fearing that he will be the fireman's next victim. However, as the two men grow to trust one another, Faber becomes a mentor to Montag, sharing insight with the fireman and conspiring with him to have copies of his books made. Faber gives Montag a small two-way radio of his own invention to insert in his ear so that the two men will always be in communication.
At home, Montag becomes disgusted with his wife and her friends as they sit idly, watching television and engaging in gossip that reveals their selfishness and lack of awareness or concern for the impending atomic war. Against Faber's objections streaming through the secret radio echoing in his ear, Montag engages the women in a debate about family and politics. Next, he reads to them from a book of poetry. Mildred's friends react emotionally to Montag's reading, crying and not understanding the source of their tears. When Mildred's two shaken friends depart, she retires to her room to take some sleeping pills and Montag hides his books in the backyard before heading off to work, where Beatty engages in more anti-book, anti-intellectual rhetoric. The firemen are called to an alarm, and Montag is dismayed to discover that it is his own house that is to be burned. His wife Millie reported him.
After burning his home and possessions by himself, room by room, as ordered by Captain Beatty, Montag is chided by his boss, and the two men engage in a scuffle, during which Faber's radio is knocked from Montag's ear. When Beatty remarks that both Montag and his "friend" (Faber) will be dealt with severely, Montag threatens him with the flamethrower. When Beatty continues to verbally abuse him, Montag flips the switch and kills the chief. At once, the Mechanical Hound, a computerized attack dog that can track down any human being by scent, pursues him. The Hound stabs him in the leg with a procaine needle, but Montag is able to annihilate it with the flamethrower before it can do more damage. Montag retrieves his remaining books from the yard before running to Faber's. On the way, he pauses to plant the books in the home of fireman Black, briefly collects himself at a gas station where he hears reports that war has been declared, and when crossing the road is nearly run over by a reckless driver.
Faber provides refuge for Montag, who is being hotly pursued by a second Mechanical Hound and the authorities. Faber provides Montag with some old clothes (masking his scent and thus impeding the Mechanical Hound), and tells him to go to the river and float downstream to the train tracks, where he will hopefully find a hobo camp of intellectual outlaws who can help him. In turn, Montag encourages Faber to turn on all of his sprinklers to throw the Hound of his scent. Montag departs, Faber heeds his advice, and then sets off for St. Louis to commission a former printer he knows to print some books. Montag floats down the river, successfully avoiding the Hound, and comes upon a group of former writers, clergymen, and academics by the riverbank. The leader of the group, an author named Granger, welcomes Montag and offers him a concoction to change his pH so that the Hound cannot detect his presence. The men then use a portable television to watch the police chase Montag's escape has caused. Montag is shocked to see the Mechanical Hound kill another man as the announcer proclaims, "Montag is dead!" The police, not wanting to lose the confidence of the public, set the Hound after an innocent man when it lost Montag's scent.
Granger tells Montag how the men in his camp have each memorized literary works so that someday, when it is safe to do so, they can again print books, recreating them from memory. When atomic bombs destroy the city, the men set out to sift through the rubble and begin anew. They plan to foster a society where books and free thought can flourish.