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In 1937, during the Spanish Civil War, American teacher-turned-Republican soldier Robert Jordan reluctantly keeps his promise to kill his compatriot Kashkin after he is wounded while escaping from a Nationalist troop train they have blown up, so that Kashkin will not be captured.  Robert, known as "Roberto" by the Spaniards, then goes to Madrid to report to Republican General Golz.

In the midst of an air raid, Golz assigns Robert to blow up a strategic bridge at dawn, the exact moment the Republicans are to launch a surprise air assault.  Golz gives Robert three days to prepare, and sends him to hide in the mountains with an older Spanish guide named Anselmo.  After spying on the Nationalist outpost near the bridge, Anselmo leads Robert to a mountain cave, which serves as a hideout for a small band of guerrilla soldiers and Gypsy refugees led by a man named Pablo.  Robert hides the dynamite in the cave and meets his new compatriots:  Pablo, a relentless freedom fighter now reduced to a surly drunkard; Pilar, Pablo's homely but courageous wife; Rafael, a cheerful gypsy; Primitivo; Andres; Fernando; and Maria, a young Spanish refugee who was brutalized by the Nationalists after they murdered her parents, and was then rescued from a prison train by Pablo and his gang.

Robert seeks the gypsies' help to carry out his orders, but, fearing a Nationalist reprisal, Pablo refuses.  Pilar then frankly informs Robert that she no longer trusts Pablo, and rallies the others to back her.  Late that night, Maria warns Robert to be wary of Pablo, who is now hostile because Pilar has taken command of the little group.

The next morning, Fernando reveals that he had sneaked away from camp to spend the night in town with his wife, and overheard Nationalists discussing rumors of a Republican attack at the bridge.  Pilar, Maria and Robert hike through the mountains to meet with the rebel El Sordo, a gypsy renegade, who promises to steal the horses they will require for their escape after the bombing.  Sensing the attraction between Robert and Maria, Pilar purposely leaves them alone together, and they fall in love.  A snowstorm blows in that night, and everyone worries that Nationalist guards scouting the mountains may follow the tracks of El Sordo's stolen horses to their outpost.  Pablo's drunken ravings prompt his compatriots to throw him out, even though they need his intimate knowledge of the mountains to help them escape after the attack.  After Pablo leaves, Pilar tells Robert that Pablo was not always a coward, and that when the war broke out, he was a fearless leader.

Noting that he organized the citizens against a Nationalist attack, Pilar recounts how Pablo saved their town.  One morning, Pablo blows up the wall around city hall after the Nationalists refuse to surrender, then mercilessly shoots four guards who approach him with their hands up.  Pablo takes over the building and sends out city officials one by one to the angry mob of local citizens.  All of the officials are brutally attacked by the mob and then thrown off a high cliff to their deaths.  Pilar is sickened by the savagery of her countrymen and refuses to take part.

Back in the present, Pilar recalls that when Pablo allowed the mob into the building, they killed the rest of their prisoners.  When Pablo returns to the cave, he has a changed attitude and tells Pilar that he regrets his past brutality, and if he could, would restore life to every man he killed.  Now sober, Pablo pledges his support for the bridge attack.

The next day, Robert spots a Nationalist cavalryman near the cave and shoots him.  The dead soldier's troop appears, and El Sordo's gang intercepts them and defends the mountain outpost until they are killed by fighter planes.  While Pablo is alone in the cave, he steals Robert's ignition unit and throws it in the fire.  When Anselmo reports that Nationalist troops at the bridge are being fortified, Robert realizes that they have learned of the imminent attack, and sends Andres across enemy lines with a message for Golz to stop the attack.

That night, after Maria confesses to Robert that Nationalist soldiers raped her the night they murdered her parents, she and Robert consummate their love.  Before dawn, Pilar discovers Pablo's sabotage, and Robert starts to assemble a new charge unit with a hand grenade.  Pablo confesses that his treachery was inspired by his fear of death, as he discovered that El Sordo's gang had been beheaded after being killed.  Pablo now wholeheartedly supports their efforts.

Andres, meanwhile, is sent through a long chain of command before he is able to reach Golz.  As Robert's motley troop assembles at their posts near the bridge, Golz receives the message, but the planes have already departed to attack the bridge outpost.  As the attack begins, Pilar fights alongside the men, while Pablo and his three new recruits kill the men at the guardhouse, and Anselmo reluctantly kills a sentry he knew from his village.   Robert sets the makeshift explosive in the bridge girders.  When tanks start pulling in, Rafael boldly drops a grenade inside a tank, but is killed shortly after it explodes.  As the Nationalist tanks close in, Anselmo refuses to pull the cord to explode the dynamite because Robert is still on the bridge.  Robert makes a desperate dash to pull the cord, and the resulting explosion not only destroys the bridge and a tank, but also kills Anselmo.

The gypsy troupe escapes into the mountains, where Maria waits with the horses.  Pablo arrives after cold-bloodedly killing his three recruits, so that there will be enough horses for everyone, not realizing that three of his group already have been killed.  When they are forced to gallop past enemy fire, everyone escapes unharmed, but Robert's leg is broken when his wounded horse falls on him.  Knowing he will be unable to complete the ride to the city of Gredos, Robert arranges for Pilar and Pablo to leave him behind with a machine gun to cover their escape.  Maria refuses to leave his side until he assures her that his spirit will live on with her.  As they leave Robert behind, Maria becomes hysterical and proceeds only because Pilar forces her.  Alone, Robert struggles to remain conscious by thinking of Maria, and as the soldiers approach, he ensures her safety by gunning them down.