A young man and an older man meet in the mountainside and discuss a bridge in the distance. The young man, Robert Jordan, is an American Spanish teacher fighting for the Spanish Republican forces in the Spanish Civil War, and the older man is Anselmo, his guide to the mountain region. Two nights before, Golz, a Soviet officer working for the Republicans, ordered Jordan to blow up the bridge, thus preventing the Republicans’ opponents, the Fascists, from entering the area and quelling their offensive. Anselmo introduces Jordan to Pablo, the “boss” of the group of guerilla fighters to which Anselmo belongs. Jordan immediately takes a disliking to Pablo, who is sullen and uncooperative.
Pablo and Anselmo lead Jordan to their camp, where Jordan encounters Maria, a member of the guerilla group. She is beautiful, despite her cropped haircut, and he is struck by her. Shortly thereafter, it is revealed that Maria had her head shaved as a prisoner in Valadolid, and that the guerillas rescued her from the wreckage of a blown-up train. Rafael, another one of the guerillas, recalls how traumatized Maria was when they picked her up, and how the “old woman”—Pablo’s wife—tied a rope to her to beat her with in order to keep her moving through the mountains toward their camp. Jordan meets Pilar, the mujer (“woman”) of Pablo; she is a stern, authoritative woman who seems to hold more power over the group than her husband. She reads Jordan’s palm but refuses to tell him what she foretells. Later, while surveying the bridge in preparation for the attack, Jordan and Anselmo discuss religion and the nature of killing.
Upon Jordan and Anselmo’s return to the camp, Pablo declares that he will not support the offensive on the bridge, though Pilar, who likes Jordan, agrees to his plan. Later that night, Rafael asks Jordan why he decided not to kill Pablo over his disobedience, and he tells Jordan that he will “have to kill him sooner or later.” Pablo interrupts their conversation, seemingly in better spirits, and tells Jordan he is “welcome” in the camp.
Maria and Jordan begin to develop a connection, of which Pilar takes notice; Maria visits Jordan in his sleeping bag at night, and they declare their love for each other and have sex. The group then travels through the mountains to reach El Sordo (Spanish for the “deaf one”), who leads another guerilla group, and ask for his support. Pilar tells Jordan about the brutal murder of several Fascist sympathizers in Pablo’s hometown and explains that Pablo has become disillusioned with the war and the Republican cause. El Sordo decides to help Pablo’s guerillas with the offensive on the bridge, and on the way back from his camp, Maria and Jordan have sex in the woods another time.
In an extended stream-of-consciousness section, Jordan considers his own disillusionment with the war and his lack of politics; he feels uncertain about his future, though he wants it to include Maria. Reuniting with Pilar later, Maria tells Pilar that the “earth moved” when she and Jordan were together, and Maria notes that the earth only moves three times in an individual’s life during love-making.
At the camp, Pablo has become angry and drunk, and he begins to antagonize the group, leading Jordan to consider killing him. Pablo leaves the camp, and the guerillas, including Pilar, decide that he must be killed for their own safety. Once again, though, Pablo’s mood shifts as soon as he reappears, and he decides to continue with the bridge detonation. Jordan recalls his experiences in Madrid before the war, where he met Karkov, a Soviet agent and journalist. After, Maria and Jordan meet another time; they decide that they feel united, as if they share a body.
The next morning, Jordan is awakened by an approaching fascist soldier, whom he kills. The guerillas realize that El Sordo and his guerillas, camped on a hill nearby, have been attacked by the Fascists. The novel shifts to the perspective of El Sordo and his men during the ambush, and it is revealed that the Fascists succeeded in killing El Sordo’s entire group.
Jordan writes to the Republicans and General Golz to cancel the bridge offensive, citing the Fascists’ knowledge of the guerilla groups’ locations. He thinks about his grandfather, an American Civil War veteran. Later, Maria visits him, and the two dream about a life together in Madrid. Maria tells Jordan about her sexual assault at the hands of the Falangists, members of a Fascist splinter group.
The novel then changes perspective again: Karkov, in Madrid at the Hotel Gaylord, learns from a reporter that the Fascists are “fighting among themselves,” though in fact, the Fascists have attacked El Sordo’s group. In the morning, Pilar tells Jordan that Pablo has left the camp with the explosives; despairing, she admits to Jordan that she feels she has failed him and the Republic. Andrés, the messenger tasked with delivering the letter from Jordan to Golz, travels through the hills, reminiscing on his love for bullfighting and the crises of civil war.
Meanwhile, back at the camp, Jordan lies next to Maria, silently raging about Pablo, the other guerillas, and the inanity of the war. In the hills, Andrés is stopped by Republican forces. At the same time, Jordan calms down and resolves to continue with the offensive. He and Maria make love, reveling in their last moments together. Pablo returns to the camp, claiming that he had a “moment of weakness” and revealing that he threw the explosives into a river. Jordan feels better once Pablo tells him that he has brought five men and their horses from a nearby village to join the group. Andrés is brought to Golz’s headquarters, but André Marty, a Frenchman allied with the Republicans, suspects both André and his escort, Rogelio Gomez, are undercover Fascists and has them arrested. Marty later realizes that he has made an error and delivers the message to Golz, but it has come too late, since the Republicans have begun the offensive on the bridge already.
At the bridge, Jordan and Anselmo shoot attacking Fascists and prepare for the explosion. Anselmo is killed in the blast, and several of the other guerillas are killed by Fascist gunfire. Jordan’s horse is shot by a Fascist as the remaining guerillas scramble to leave the area, and Jordan falls off of the horse. His left leg is broken as the horse lands on it. Maria and Pilar tend to Jordan, but Jordan tells them to leave and save themselves. As Jordan prepares for one final offensive on the Fascists before he succumbs to his injuries, he realizes that he has fought for what he believed in, and that though he hates “very much” to leave the world behind, he is lucky to “have had such a good life.”
