The screenwriter Dudley Nichols based his script on the 1940 novel For Whom the Bell Tolls by American novelist Ernest Hemingway. The film is about an American International Brigades volunteer, Robert Jordan (Cooper), who is fighting in the Spanish Civil War against the fascists. During his desperate mission to blow up a strategically important bridge to protect Republican forces, Jordan falls in love with a young woman guerrilla fighter (Bergman).