1943. After his wife takes their son and leaves him, Sgt. John M. Stryker is an embittered man who takes his misery out on the men under his command. They're a bunch of green recruits who have a hard time dealing with Stryker's tough drills and thicker skin. Even his old friends start to wonder if he's gone from being the epitome of the tough Marine drill instructor to a man over the edge. One recruit in particular is a source of friction - Peter Conway. Conway's father was Stryker's CO at Guadalcanal and resented his son's seeming cowardice as a Marine, while Conway resents both Stryker's brutal leadership and seeming lack of sympathy for his men as well as the resentment he Conway feels is vented against him over his more relaxed conduct as a soldier.
Another foe is Al Thomas, who was demoted by Stryker in previous action. The squad is assigned to take part in the November 1943 assault on the Japanese base at Tarawa. Stryker's brutal training regimen is now put to the test as the Marines must overcome a powerful underground bunker at the beach's edge; the bunker's machine gunners slaughter multiple Marines before Stryker succeeds in blowing up the position with satchel charges, and his bravery does not go unnoticed by his men. When Al Thomas and two other Marines, Charlie Bass and Hellenopolis, become separated from the main unit, Thomas runs to get fresh ammo for them - but rests for a moment when a Marine give him fresh hot coffee, a lapse that allows his two buddies to be jumped and bayoneted by Japanese Naval Landing troops. That night the beaten-up squad must guard the main approach to the beachhead, and they hear in the distance a man desperately calling for help.
Stryker believes it to be a deception by the Japanese and he can't let anyone tip their position - which leads to an even grimmer dilemma when the voice calls out, "Stryker!" Peter Conway violently vows to find the man - but Stryker nearly shoots him dead to keep him from moving, and must sweat out the possibility of letting a fellow Marine die. The island is finally secured and wounded found and returned. Months later Charlie Bass returns to duty and queries about what happened to Al Thomas. Stryker then confronts Thomas and a fight erupts - broken up by a Marine Colonel. Thomas for his part profusely confesses he did wrong - and that he'd prefer to be dead himself than to have let his buddies die.
Seeing Thomas' sincerity, Stryker is able to forge forgiveness. Turning to the bottle for comfort Stryker confronts his demons head on but then shows his soft side and helps out a widowed military wife and her child. Then the word comes down that they are being sent to Iwo Jima to do battle with 20,000 Japanese who've been dug into the most powerful island fortress of the war - a fortress so powerful that the largest continuing naval bombardment in history cannot dislodge the Japanese after three vicious days of combat, during which Conway saves Stryker's life when he is jumped by a Japanese soldier with sword. Finally Marines begin to advance up the island's largest rampart, Mount Suribachi, amid continuing fire from the US Fleet. The Marines reach the summit and Stryker sends four Marines to set up the US flag.
But as Stryker now seems at peace, he is shot by a sniper, and Conway, Thomas, Bass, and the other Marines can only find and read an unfinished letter by Stryker to his son - just as the Marines raise the US flag atop the mountain..