For Whom the Bell Tolls is set in 1937 during the Spanish Civil War, a struggle between those in power (the Republicans or Loyalists) and those trying to depose the government, the Fascists. The Republicans are supported by the Soviet Union, and the Fascists, which are winning the war, are supported by Nazi Germany. It is an era where there was considerable support for the Soviet experiment; thousands of idealistic Americans went to Spain and fought with the Republicans.

Robert Jordan, an American fighting on the side of the Republicans, has been assigned to blow up a mountain bridge in the north of Spain to cut off Fascist reinforcements in preparation for the launch of a much awaited Republican offensive. Under the guidance of Anselmo, Robert Jordan connects with a band of guerrillas that he has been told will help him in his mission. Instead he discovers that they are lead by the drunken Pablo who opposes Robert Jordan’s mission for fear the Fascists who know where they are will hunt them down, forcing the small band to abandon the safety of their cave.

Supporting Robert Jordan is Pilar who asserts her leadership of the restless guerrillas who support her over Pablo’s shiftless refusal to take action. Robert tries to avoid becoming enmeshed in their internal struggles, aware he has become disillusioned in the potential for success of their cause – and in his mission. Still he is almost immediately taken by the engaging, though guileless Maria, who for all of her traumatized past, finds herself equally drawn to him, recognizing they are both looking for something or someone to believe in and live for.

The logistics for blowing up the bridge start falling into place when El Sordo, leader of a nearby band of guerrillas, agrees to steal the horses they need to make their escape afterwards. Nevertheless Robert Jordan learns that word has gotten out about the planned Republican offensive, compromising their success.

At the start of Act Two, matters take a turn for the worse when Fascist militia follow El Sordo’s tracks in the newly fallen snow back to his hiding place and wipe out his band of guerrillas. That night, Pablo out of self-preservation steals the detonators Jordan needs for his explosives, and disappears. Robert Jordan realizes that although he can use grenades to trigger the dynamite for blowing up the bridge later that morning, it has become a suicide mission without sufficient men to attack the heavily guarded bridge and still escape safely. His greatest regret is that Maria, with whom he has fallen in love, may lose her life in the fighting.

In the dawn, as they prepare to leave, Robert Jordan, Pilar and the rest of the band are joined by the mercurial Pablo who has not only recruited more men but secured the needed horses. He confesses in a moment of cowardice that he threw the detonators away but has also realized that the job can be done with grenades, though it makes the job riskier.

As everyone waits for the bombardment to begin signaling the start of the Republican offensive, Jordan assures a worried Maria that they will be together in America before too long. As she leaves to help guard the horses, Jordan and Anselmo prepare their assault on the bridge as Pablo and Pilar lead separate groups of men in attacking the guardhouses positioned at each end of the bridge.

The mission is a success but not without losses including Anselmo who is killed. As they all gather once more after the bridge has been blown up, Pablo turns his submachine gun on the men he has recruited, so his people will have enough horses for their escape.

As they make their dash to safety in a hale of enemy gunfire, Jordan is wounded. He realizes he cannot go on without threatening everyone else’s ability to get away. He takes Maria aside and assures her that he will be with her always and that wherever she goes, he will be with her. Once the rest have left, Jordan takes up his position with a machine gun to fight off the Fascists to the very last, while the others – including Maria – escape capture.