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.