The novel graphically describes the brutality of the civil war. It is told primarily through the thoughts and experiences of the protagonist, Robert Jordan. The character was inspired by Hemingway's own experiences in the Spanish Civil War as a reporter for the North American Newspaper Alliance.
Jordan is an American in the International Brigades who travels to Spain to fight for the Republic in opposition of Francisco Franco's fascist forces. An experienced dynamiter, he is ordered by a Soviet general to travel behind enemy lines and destroy a bridge with the aid of a band of local anti-fascist guerrillas, in order to prevent enemy troops from responding to an upcoming offensive. On his mission, Jordan meets the rebel Anselmo who brings him to the hidden guerrilla camp and initially acts as an intermediary between Jordan and the other guerrilla fighters.
In the camp, Jordan encounters María, a young Spanish woman whose life had been shattered by her parents' execution and her rape at the hands of the Falangists (part of the fascist coalition) at the outbreak of the war. His strong sense of duty clashes with both the unwillingness of the guerrilla leader Pablo to commit to an operation that would endanger himself and his band, and Jordan's own new-found lust for life which arises from his love for María. Pablo's wife, Pilar, usurps Pablo's leadership and pledges the allegiance of the guerrillas to Jordan's mission. However, when another band of anti-fascist guerrillas, led by El Sordo, is surrounded and killed, Pablo steals the dynamite caps, hoping to prevent the demolition and thereby avoid fascist reprisals. In the end, Jordan improvises a way to detonate his dynamite and destroy the bridge. Pablo regrets abandoning his comrades and returns to assist in the operation.
However, the enemy, apprised of the coming offensive, has prepared to ambush it in force and it seems unlikely that the blown bridge will do much to prevent a rout. Regardless of this, Jordan understands that he must still demolish the bridge in an attempt to prevent Fascist reinforcements from overwhelming his allies. After Pablo loses the dynamite caps, Jordan and Anselmo coordinate an alternative method to destroy the bridge which is much more dangerous because of the men's proximity to the explosives. While Pablo, Pilar, and Maria create a distraction for Jordan and Anselmo, the two men set and detonate the explosives, costing Anselmo his life when he is hit in the face by a piece of shrapnel. While escaping, Jordan is maimed when a tank shoots his horse out from under him. Knowing he would only slow his comrades down, he bids goodbye to María and ensures that she escapes to safety with the surviving guerrillas. He refuses an offer from another fighter to shoot him and lies in agony, hoping to kill an enemy officer and a few soldiers, and delay their pursuit of his comrades before dying or being killed. The narration ends right before Jordan launches his ambush.