This novel traces three days in the life of Robert Jordan, an American Spanish professor who has volunteered to fight for the Loyalists in the Spanish Civil War. Jordan is a dynamite expert, and is ordered by General Golz, a Russian leader of the International Brigades, to bomb a bridge as part of their offensive against the Fascists. Golz is only interested in the offensive as a means of practicing his military tactics and he is cynical about its success in the hands of the Spanish peasants.
Anselmo, an old guide, brings Jordan through the woods to the hideout, an abandoned cave, of the men who will help him complete his mission. The guerillas that Jordan encounters obviously do not want to be involved in the war any longer. They meet Agustin in the woods, visibly relieved to see them because he has forgotten the password to their lair. The gypsy Rafael, despite being the guard, is only interested in cracking jokes. He tells Jordan about Kashkin, the previous foreign dynamite expert who, ironically, killed himself after being wounded during their last mission, the explosion of a train. The most cynical and despondent guerilla, however, is Pablo, their leader. Despite being a courageous man before, Pablo now wants only to return to his village to raise the horses he gained as spoils of war. Many conflicts arise between Pablo and Jordan, as the Pablo resents that a foreigner is interfering in a matter that can risk his own life and those of his band.
There are also two women at the camp: Pilar, who is Pablo's wife, and Maria, a girl they rescued from the train carrying prisoners of war. Despite her cropped hair, which was shaved during her interment by the Fascists and the obvious psychological damage wrought upon her, she is beautiful. Pilar is an ugly woman, but celebrated for her bravery. Since Pablo "went bad" and lost the courage and zeal he displayed at the beginning of the war, Pilar maintains the unity of his band. Pilar is a gypsy and, upon introductions, reads Jordan's palm. The future she foretells there, but will not reveal, is grim.
Pablo's cowardice soon makes him relinquish power to Pilar, his bold wife. Pablo announces that he is against blowing up the bridge, but Pilar backs Robert Jordan and the men follow her lead. After the confrontation, Rafael tells Jordan that he should have killed Pablo, and that he would have had the support of the guerillas. Jordan reasons that, unprovoked, this would be assassination. As Pablo continues to insult and cause trouble of Jordan throughout the novel, Jordan wonders if he made the right decision.
After the confrontation with Pablo, during the night after the first day, Jordan makes love to Maria when she comes to his makeshift bed outside the cave. The nineteen-year-old girl, who has been raped and orphaned, has fallen quickly and madly in love with Jordan. She believes that her love will purify her from past atrocities committed to her. Jordan returns her feelings, as he has gazed upon her all day with a lump in his throat. He celebrates finding, for the first time, happiness in unity with another individual.
Jordan's newfound love, however, is overshadowed by the many obstacles he must face to complete his mission. The appearance of enemy planes, for one, heighten tension at the camp because either they are planning an attack of their own, or have gotten wind of the Loyalist offensive. So too, when Maria, Pilar and Jordan journey up the mountain to the guerilla leader El Sordo's camp, he reminds them of how dangerous the bridge mission is. He agrees to help them, but as they leave camp it begins to snow. Now, the enemy could be able to follow El Sordo's tracks to the bridge.
The only person who really encourages Jordan is Anselmo, who he finds loyally waiting in his post, despite the storm, for Jordan to dismiss him. Besides being a loyal soldier who is committed to the Cause, Anselmo is distinguished as a true humanitarian. He is preoccupied not with the thought of losing his own life during the attack on the bridge, but rather fears that Jordan will order him to kill another human being. He sees the enemy not as evil Fascists, as do the others, but as poor countrymen like themselves.
Pablo again makes trouble for Jordan on the second day, when he baits him about his relationship with Maria. Jordan tries to goad him into fighting, as this would be an appropriate time to kill him for the sake of the mission. Pablo refuses to be baited, however, and later resumes a cooperative mood. Jordan trusts him less than ever, and grows increasingly worrisome about the success of the mission. Thus, Jordan feels his time is limited, which is evidenced by his urgent need to make love to Maria.
The next morning, Jordan is awakened by the sounds of an approaching enemy horseman. Jordan shoots the soldier, and the camp frantically scrambles to arm themselves with a machine gun that did not even come with directions. Tension mounts as Fascist troops pass by the camp. Jordan acts as the example of level-headedness for his men, as Agustin wants to kill the passing soldiers. Then, sounds come from El Sordo's. His camp is attacked and bombed, and they all are killed. Primitivo urges Jordan to help El Sordo, but Jordan knows that the bridge mission must be his priority, even over the lives of his comrades. Thus, the guerillas remain undiscovered for the time being. The fighting between El Sordo and the Fascists, led by Lieutenant Berrendo, show how neither side really wants to fight or die. Jordan sends a young guerilla, to General Golz with news of El Sordo's defeat and a request that the offensive be cancelled.
The last night before the attack is very eventful. Maria is inflicted by pain, so the couple discusses their future and their luck in finding each other. Jordan, however, thinks that being unable to make love is a bad omen. Indeed, his presentiment comes true when Pilar wakes him with the news that Pablo, ever treacherous, has fled with some dynamite.
Jordan is worried now that his plan won't work. Jordan does not have enough men and Pablo stole the equipment he needed to blow the bridge correctly. It is highly unlikely that the attack will be postponed, even if Andres does deliver the message to General Golz. Pablo returns that morning accompanied by five extra men and their horses, claiming that he is not a coward after all and will help blow the bridge.
The apathy and inefficiency of the Loyalist army stalls Andres, and the message does not reach General Golz in time. The bridge bombing must proceed. At the bridge, Jordan orders Anselmo to kill the sentry, which he tearfully accomplishes. Then they dynamite the bridge, and Anselmo is killed by a falling rock. In the ensuing fighting, the only guerillas who survive are Pablo, Pilar, Maria, Primitivo and Agustin . Jordan is hit by a shell as they escape on horseback and is unable to escape. He tells Maria that they will always be one person, and refuses to be shot out of mercy. His comrades give him a machine gun so that he can defend himself from the approaching enemy. Jordan fights pain and suicidal thoughts with the hope that he can buy time for the fleeing guerillas. The novel closes here, as Jordan awaits his certain death on the pine-covered ground he appeared on in the first scene.