The novel chronicles the experiences of American professor Robert Jordan from Saturday afternoon to Tuesday noon during the last week of May 1937. Jordan has volunteered to fight with the Loyalist guerrilla army in the Spanish Civil War. His mission is to blow up a bridge near Segovia prior to a Loyalist offensive in that area, scheduled to occur in three days. When the novel opens, he is behind enemy lines, ready to meet up with Pablo and his wife Pilar, his contacts, and the leaders of one of the guerrilla factions.

Jordan studies the bridge as he determines how he will blow it up at the necessary moment. He has previously blown up bridges and trains, but he never has had to time a demolition so carefully. Pablo and Pilar have been set to help Jordan plan and execute the mission, gathering together other guerrilla bands...