During World War I, Lt. Frederic Henry, an American Red Cross ambulance driver assigned to an Italian unit at the battlefront, returns from his leave in Milan to regale his womanizing doctor friend, Maj. Alessandro Rinaldi, with stories of his conquests. Rinaldi retorts that the English have opened a new hospital staffed with beautiful nurses, among them the enigmatic Catherine Barkley. Henry meets the morose Catherine and becomes intrigued when she confides her regrets about not marrying her fiancé before he died in battle. Just before his battalion is to launch an offensive, Henry visits Catherine, who slaps him when he caresses her. When it starts to storm, Catherine, terrified of the rain, begins to sob and after Henry comforts her, they fall into a passionate embrace. After a night of lovemaking, Henry pledges his love but Catherine remains noncommittal. As the troops file out of town, Henry scours the crowd for Catherine, who, experiencing a change of heart, threads her way through the throngs to meet him. After they embrace, Catherine begs Henry to return to her. The ambulances follow the troops on their long trek through the snow-covered mountains, preparing to whisk the injured back to the hospital. When the shelling starts, two ambulances are destroyed and Henry is badly wounded in the knee. His friends hurriedly load him into the remaining ambulance and speed down the mountain to the hospital. Henry is to be sent to the new American hospital in Milan, and so Rinaldi arranges for Catherine to be transferred there, too. The hospital's first patient, Henry is attended to by the stern head nurse, Miss Van Campen, and the more sympathetic Helen Ferguson. On the day that Catherine finally arrives, Henry reassures her of his love and proposes. Aware that as a military wife, she will be sent away from the front, Catherine rejects his proposal and asserts that she does not need marriage. Although Ferguson is cynical about wartime romances, she helps conceal Henry and Catherine's affair from Van Campen. When Catherine informs Henry that she is pregnant, he insists on marriage, but she refuses once again and instead they pledge their vows to each other. Soon after, Van Campen discovers Catherine in Henry's hospital bed and indignantly notifies headquarters, assuring that Henry will be sent back to active duty that night. After one final fling in a hotel room, they tearfully bid farewell. Back at the front, Henry finds Rinaldi a broken man, the victim of trying to piece together too many mangled soldiers. The ambulances are dispatched to Capretto, but as they approach the city, they find the area in ruins, its houses bombed, its people fleeing in panic and the Italian army in hasty retreat from the savage Germans. With the Germans in pursuit, the doctors and ambulances are ordered to leave the hospital patients behind and accompany the retreating troops. Henry begs his friend Father Galli to join them, but the priest elects to face certain death and remain behind with the patients. The fleeing masses find the roads littered with the dead bodies of soldiers, women and small babies. When the ambulance loses an axle and breaks down, Rinaldi starts to rant about surrender and is arrested as a German spy sent to undermine morale. After Rinaldi is found guilty and sentenced to die, Henry pleads in vain for his friend's life as Rinaldi is hauled in front of the firing squad. When the court then challenges his identity, Henry deserts, plunging into the river. Taking the clothes from a corpse he finds floating in the water, Henry hops a train back to Milan. Upon finding Catherine, Henry declares that he is through with war and has made a separate peace. Deciding to run away to Switzerland, they brave the crossing in a small rowboat. After being battered by a storm and nearly being detected by a patrol boat, they reach the border and meet Lt. Zimmerman, a Swiss police officer, who sends them to his mother's hotel in the mountains. Six weeks later, Catherine, visibly pregnant, refuses to wed Henry for fear of creating a scandal in the village. With the coming of spring, Catherine goes into labor. After a protracted, painful labor, Dr. Emerich advises performing a caesarian, and soon after, delivers a baby boy. Over dinner that night, Emerich gently informs Henry that his son has died, and Henry declares that his son's death is punishment for his own war crimes. Back at the hospital, Henry learns that Catherine has begun to hemorrhage, and prays for her life. As he clasps her hand, Catherine dies and Henry promises that she will be with him forever. He then leaves the hospital, absorbed in his memories of Catherine.
