Guy Montag is a fireman who believes he is content in his job, which, in the oppressive future American society depicted in Fahrenheit 451, consists of burning books and the possessions of book owners. However, his discontent, secret even from himself, becomes clear after he meets Clarisse McClellan, a teenage girl and his new neighbor, who engages in such outlandish behavior as walking instead of driving and having conversations. She asks him if he's happy. When he returns home to find that his wife, Mildred, has taken a bottle full of sleeping pills, he realizes that he is not happy. Mildred is saved, but the next day she has no memory of her suicide attempt. She sits in the parlor, engrossed in its three full walls of interactive TV.
Back at the fire station, Montag is threatened by the Mechanical Hound, a robotic hunter that can be programmed to track any scent. Captain Beatty tells him not to worry—unless, Beatty adds jokingly, Montag has a guilty conscience. For the next week, Montag continues to talk with Clarisse and to examine his own life. One day, while the radio in the fire station mentions that war is imminent, Montag asks Beatty if there was a time when firemen prevented fires, instead of started them. The alarm rings, and the firemen all head to the house of an elderly woman whose neighbor has turned her in. The woman refuses to leave her house as they douse it in kerosene. She lights a match herself and burns along with the house.
In bed that night, Montag asks Mildred—who, as usual, is zoning out listening to her earbud radio—where they met. Neither of them can remember. Mildred tells Montag that Clarisse has been killed. Haunted by the vision of the old woman's death, and by the news of Clarisse's death, Montag doesn't go to work the next day. Beatty visits him at home and delivers a long lecture on the history of censorship, the development of mass media, the dumbing down of culture, the rise of instant gratification, and the role of firemen as society's "official censors, judges, and executors." Beatty says it's okay for a fireman to keep a book for 24 hours out of natural curiosity, so long as he turns it in the next day. When Beatty leaves, Montag shows Mildred twenty books, including a Bible, that he's been hiding in the house. He feels that their lives are falling apart and that the world doesn't make sense, and hopes some answers might be found in the books. Montag and Mildred try to read the books.
But reading is not easy when you have so little practice. Mildred soon gives up and insists that Montag get rid of the books so they can resume their lives. Montag, however, remembers a retired English professor named Faber whom he met a year ago and who might be able to help. On the subway trip to the man's house, Montag tries to read and memorize passages of the Bible he's brought with him. Faber is frightened of Montag at first, but eventually agrees to help Montag in a scheme to undermine the firemen. They agree to communicate through a tiny two-way radio placed in Montag's ear. When Montag returns home, his wife's friends are over watching TV. Montag loses his cool. He forces the women to listen to him read a poem by Matthew Arnold from one of his secret books. They leave, greatly upset. When Montag goes to work, Beatty mocks him with contradictory quotations drawn from famous books, which point out that books are useless, elitist, and confusing. Montag hands over a book to Beatty and is apparently forgiven. Suddenly, an alarm comes in. The firemen rush to their truck and head out to the address given. It's Montag's house.
As they arrive, Mildred leaves the house and ducks into a taxi. She is the one who called in the alarm. Beatty forces Montag to burn his house with a flamethrower, and then tells him he's under arrest. Beatty also discovers the two-way radio and says he'll trace it to its source, then taunts Montag until Montag kills him with the flamethrower.
Now a fugitive and the object of a massive, televised manhunt, Montag visits Faber, then makes it to the river a few steps ahead of the Mechanical Hound. He floats downstream to safety. Along some abandoned railroad tracks in the countryside, Montag finds a group of old men whom Faber told him about—outcasts from society who were formerly academics and theologians. They and others like them have memorized thousands of books and are surviving on the margins of society, waiting for a time when the world becomes interested in reading again. Montag is able to remember parts of the Book of Ecclesiastes, so he has something to contribute.
Early the next morning, enemy bombers fly overhead toward the city. The war begins and ends almost in an instant. The city is reduced to powder. Montag mourns for Mildred and their empty life together. He is at last able to remember where they met—Chicago. With Montag leading, the group of men head upriver toward the city to help the survivors rebuild amid the ashes.