At a New Zealand Marine base in 1943, Corporal Robert Dunne recounts his first tour of duty: Before Dunne and his friends, Italian-American Benny Regazzi, Greek-American George Hellenopolis, Polish-American "Ski" Choynski, Jewish-American Sid Stein, two Irish-American brothers named Flynn, and Dan Shipley and Charlie Bass, meet their battle-hardened commander, Sgt. Stryker, they learn of his infamous toughness and mysterious demotion. One morning early in the training period, Stryker discovers that Peter Conway, the son of his deceased former commanding officer, Col. Sam Conway, has joined his unit. Although Stryker attempts to bond with Pete over Sam's memory, Pete reveals that he hated his demanding father and also despises Stryker for his resemblance to the man. When the mail arrives later, Stryker is saddened to find no letter from his own son, of whom his wife took sole custody when she left him five years earlier. After finally receiving some leave time, Stryker's men decide to visit a dance hall. There Pete meets and falls in love with Allison Bromley and Stryker drinks to excess. A few days later, Stryker urges Pete not to become too attached to Allison and then sympathetically overhears him discussing his desire to procreate in the face of all the death around him. Only a day after Pete and Allison marry, the unit is sent to the Tarawa atoll, which is occupied by Japanese forces. After Stryker's unit lands, he orders two of his men to cross a dangerous minefield and place a charge inside a bunker where the Japanese have hidden some explosives. Stryker watches as both men are shot down, then rushes forward to grab the charge and complete the mission single-handedly. Later, Corp. Al Thomas decides to take a coffee break in the mortar men's foxhole, while leaving his subordinates, Bass and Hellenopolis, alone in theirs. This move inadvertently causes Hellenopolis' death and Bass's wounding. The unit is then ordered to entrench, without movement, in an unsafe zone, and despite the pleas of his men to rescue the nearby Bass, Stryker refuses to violate his orders, and they are forced to listen to Bass's plaintive cries for help until the next morning, when Bass is rescued. Later, the unit arrives in Hawaii, where Bass tells Stryker that Thomas was absent from his post during the attack. Stryker immediately starts a fight with Thomas, and when a major demands to know who started it, Thomas, realizing that another demotion could end Stryker's career, protects his sergeant. He then tells Stryker of his deep guilt and remorse, after which Stryker forgives him. Soon after, Pete learns that Allison has given birth to a son, news which secretly pleases Stryker. While on leave in Honolulu, Stryker goes home with a woman forced into prostitution to care for her baby son and realizes that he has been wallowing in self-pity over the loss of his family for too long. Later, during grenade training, Stryker saves Pete's life. Soon after, the unit ships out and is ordered to complete a difficult landing on the rocky, island cliffs of Japan's Iwo Jima. The fighting is intense, with many of the unit's men dying, and although Pete is afraid he will die, he saves Stryker's life and reveals to him that he plans to name his son Sam after his father. When the unit finally gains the top of the island's Mount Suribachi, Stryker instructs his men to hoist the American flag, but is killed by a random bullet. Pete drops to his knees to embrace Stryker's corpse and finds an unfinished letter that he had been writing to explain himself to his son. With fond remembrance of his brave commander, Pete completes the letter.