For Whom The Bell Tolls opens in May 1937, at the height of the Spanish Civil War. An American man named Robert Jordan, who has left the United States to enlist on the Republican side in the war, travels behind enemy lines to work with Spanish guerrilla fighters, or guerrilleros, hiding in the mountains. The Republican command has assigned Robert Jordan the dangerous and difficult task of blowing up a Fascist-controlled bridge as part of a larger Republican offensive.
A peasant named Anselmo guides Robert Jordan to the guerrilla camp, which is hidden in a cave. Along the way, they encounter Pablo, the leader of the camp, who greets Robert Jordan with hostility and opposes the bridge operation because he believes it endangers the guerrilleros’ safety. Robert Jordan suspects that Pablo may betray or sabotage the mission.
At the camp, Robert Jordan meets Pilar, Pablo’s “woman.” A large, sturdy part-gypsy, Pilar appears to be the real leader of the band of guerrilleros. A rapport quickly develops between Robert Jordan and Pilar. During the course of the evening, Robert Jordan meets the six other inhabitants of the camp: the unreliable Rafael, feisty and foul-mouthed Agustín, dignified Fernando, old Primitivo, and brothers Andrés and Eladio. The camp also shelters a young woman named Maria, whom a band of Fascists raped not long before. Robert Jordan and Maria are immediately drawn to each other.
Robert Jordan and Anselmo leave the camp to scout out the bridge. When they return, Pablo publicly announces that neither he nor his guerrilleros will help blow up the bridge. Pilar and the others disagree, however, so Pablo sullenly gives in. Privately, Rafael urges Robert Jordan to kill Pablo, but Pilar insists that Pablo is not dangerous. That night, Maria comes out to join Robert Jordan as he sleeps outside. They profess love for each other and make love.
The next morning, Pilar leads Robert Jordan through the forest to consult with El Sordo, the leader of another band of guerrilleros, about the bridge operation. They take Maria along. El Sordo agrees to help with the mission, but both he and Robert Jordan are troubled by the fact that the bridge must be blown in daylight, which will make their retreat more difficult. On the way back to Pablo’s camp, Robert Jordan and Maria make love in the forest. When they catch up with Pilar, Maria confesses to Pilar that the earth moved as they made love. Pilar, impressed, says that such a thing happens no more than three times in a person’s lifetime.
Back at the camp, a drunken Pablo insults Robert Jordan, who tries to provoke Pablo, hoping to find an excuse to kill him. Pablo refuses to be provoked, even when Agustín hits him in the face. When Pablo steps away for a few minutes, the others agree that he is dangerous and must be killed. Robert Jordan volunteers to do it. Suddenly, Pablo returns and announces that he has changed his mind and will help with the bridge. Later that night, Maria comes outside to sleep with Robert Jordan again. They talk about their feeling that they are one person, that they share the same body.
In the morning, Robert Jordan wakes up, sees a Fascist cavalryman, and shoots him, awakening the camp. After breakfast, the group hears sounds of a fight in the distance, and Robert Jordan believes that the Fascists are attacking El Sordo’s camp. Agustín and Primitivo want to aid El Sordo, but Robert Jordan and Pilar know that it likely would be useless.
The scene shifts to El Sordo’s hill, which a group of Fascists is assaulting. El Sordo’s men play dead and manage to shoot the Fascist captain, but several minutes later, Fascist planes bomb the hilltop and kill everyone in El Sordo’s band. The ranking Fascist officer orders the beheading of all the corpses of El Sordo’s men.
The guerrilleros at Pablo’s camp, having heard the planes bomb El Sordo’s hill, feel glum as they eat lunch. Robert Jordan writes a dispatch to the Republican command recommending that both the bridge operation and the larger offensive be canceled, for the Fascists are aware of the plan and the operation will not succeed. He sends Andrés to deliver the dispatch to the headquarters of General Golz, a Republican leader. Maria again joins Robert Jordan in his sleeping bag that night, and they fantasize about their future life in Madrid.
Meanwhile, in Madrid, Robert Jordan’s friend, a Russian journalist named Karkov, learns that the Fascists know about the offensive the Republicans have planned for the next day. Karkov worries about Robert Jordan.
At two in the morning, Pilar wakes Robert Jordan and reports that Pablo has fled the camp with some of the explosives that were meant to blow the bridge. Though furious at first, Robert Jordan controls his anger and plans to carry out the operation anyway, with fewer explosives. He wakes up Maria, and as they make love, they feel the earth move again. Pablo suddenly returns just before dawn, claiming that he left in a moment of weakness. He says that he threw the explosives into the river but felt great loneliness after doing so. He has brought back five men with their horses from neighboring guerrilla bands to help. The fighters take their positions.
The scene shifts to Andrés, who has been traveling through the night to deliver Robert Jordan’s dispatch to General Golz. Crossing into Republican territory, Andrés is slowed when several suspicious but apathetic officers question him. When Andrés and his escort finally near Golz’s headquarters, a politician named André Marty suspects that they are Fascist spies and orders them arrested. Robert Jordan’s friend Karkov hears about the arrests and uses his influence to free the men. Robert Jordan’s dispatch finally reaches Golz but arrives too late. The Republican offensive already has begun and can no longer be stopped.
As dawn breaks, Robert Jordan and Anselmo descend on the bridge, shoot the Fascist sentries, and plant the explosives. Pilar arrives and says that Eladio has been killed, while Fernando, fatally wounded, must be left behind. When Robert Jordan detonates the explosives, the bridge falls, but shrapnel from the blast strikes Anselmo and kills him. Pablo emerges from below, saying that all five of his men are dead. Agustín accuses Pablo of shooting the men for their horses, and Pablo does not deny it.
As the group crosses the road in retreat, a Fascist bullet hits Robert Jordan’s horse, which tramples on Robert Jordan’s left leg, breaking it. Knowing that he must be left behind, Robert Jordan says goodbye to Maria, saying that he will be with her even if she goes. Pilar and Pablo lead Maria away.
Alone, Robert Jordan contemplates suicide but resolves to stay alive to hold off the Fascists. He is grateful for having lived, in his final few days, a full lifetime. For the first time, he feels “integrated,” in harmony with the world. As the Fascist lieutenant approaches, Robert Jordan takes aim, feeling his heart beating against the floor of the forest.