In 1943, after his wife leaves him and takes their son, Sgt. John M. Stryker becomes a bitter man who directs his frustration towards his inexperienced recruits. These young Marines struggle to withstand Stryker's relentless drills and unyielding nature. Even his former comrades begin to question whether he has transformed from the embodiment of a disciplined Marine drill instructor to a man on the brink of despair.
Tension mounts particularly between Stryker and recruit Peter Conway. Conway, whose father was Stryker's commanding officer at Guadalcanal, grapples with feelings of inadequacy as a Marine. He is resentful of Stryker's harsh leadership style, which lacks empathy towards his men, and he clashes with Stryker's expectations, especially since he is perceived as more lenient in combat. Al Thomas, another recruit, also bears a grudge against Stryker for being demoted in their past assignments, adding to the discord within the ranks.
As the squad prepares for the November 1943 assault on the Japanese base at Tarawa, Stryker's grueling training is pushed to the limits. The Marines face a formidable underground bunker at the beachhead, where machine-gun fire devastates their ranks before Stryker courageously detonates the bunker with satchel charges, earning the respect of his men.
The perilous situation escalates when Thomas and fellow Marines, Charlie Bass and Hellenopolis, become cut off from their unit. In a moment of hesitation while obtaining fresh ammunition, Thomas falls prey to a Japanese ambush, resulting in a heart-wrenching loss for the squad.
That night, while guarding the beachhead, they hear a voice crying out for help in the darkness. Stryker, suspecting a trap set by the Japanese, refrains from revealing their location, creating a moral crisis when the voice calls out his name. Peter Conway, driven by a sense of duty, insists on rescuing the man—but Stryker, in a moment of desperation, nearly shoots him to prevent his reckless move, torn between risking a fellow Marine’s life and maintaining their position.
The island is ultimately secured, and the wounded are retrieved. Months later, Bass returns to duty inquiring about Al Thomas. Stryker confronts Thomas, leading to a volatile altercation broken up by a Marine Colonel. Thomas, filled with remorse, admits his cowardice and expresses a willingness to take responsibility for the loss of his comrades. Through this confrontation, Stryker finds a path to forgiveness.
Struggling with his inner demons, Stryker ultimately reaches out to a grieving military widow, revealing his softer side. As fate would have it, they receive orders to deploy to Iwo Jima, where they will confront 20,000 Japanese troops entrenched in a formidable island stronghold—one that withstands the most extensive naval bombardment in history. During these fierce battles, Conway saves Stryker's life from an attacking Japanese soldier.
The final push sees the Marines ascend Mount Suribachi amid relentless fire. As they reach the summit, Stryker instructs four Marines to raise the U.S. flag. Just when he seems to attain a sense of peace, tragedy strikes as he is shot by a sniper. In the aftermath, Conway, Thomas, Bass, and their comrades can only discover and read an unfinished letter to Stryker's son, on the day they triumphantly raise the flag atop the mountain.
