The film opens with recruits getting their heads shaved. In the next scene, Gunnery Sergeant Hartman sternly welcomes the men to boot camp. Hartman insults, punches, and chokes the recruits to intimate them, reserving his most vicious brutality for a man named Leonard Lawrence, whom he nicknames "Gomer Pyle." As the men go about their daily drills and exercises, Hartman berates Pyle for being slower than the others, humiliating him in various ways. Eventually, he places Pyle in the same squad as another recruit named Joker, in an effort to improve Pyle's conduct. When Hartman finds a jelly donut stashed in Pyle's locker, he begins punishing the rest of the platoon for Pyle's mistakes.
In the middle of the night, the entire platoon viciously beats Pyle in bed, a hazing designed to "motivate" him. Although Pyle begins to improve in his daily tasks as a Marine, he also begins to lose his grip on reality, talking to his rifle and staring vacantly during their normal exercises. On Christmas Day, Hartman explains to the men that "God has a hard-on for the Marines." Joker tells another recruit named Cowboy he's worried that Pyle may be a "Section 8," meaning he is mentally unfit for service. On the last night of boot camp, Joker finds Pyle out of his bed and sitting on a latrine with his rifle. Although Joker attempts to communicate with him, the deranged Pyle shouts the Rifleman's Creed, waking Hartman up. Hartman rushes in, and Pyle kills him, then himself, while Joker looks on in horror.
The film flashes forward to Da Nang, Vietnam, where Joker has been assigned as a combat correspondent for Stars and Stripes, along with a partner named Rafterman. Joker attends briefings with his editor, Lockhart, who wants the men to write stories that pander to supporters of the war effort. Although he discourages Rafterman from going into the field, Joker himself wants to go into battle so that he can report on it. Other soldiers mock Joker for not having the "thousand yard stare." After the Tet Offensive begins, Lockhart sends Joker to Phu Bai, and forces him to take Rafterman along, against Joker's wishes.
On the way to Phu Bai, Joker listens to a helicopter gunner describe killing women and children. He later finds a mass grave, filled with corpses covered in lye. After finding a squad leader named Touchdown, Joker reunites with Cowboy, who has become part of a group named the "Lusthog Squad." Other members of the Lusthog Squad include Animal Mother, a hulking man who wears ammunition belts, and Crazy Earl, an ardent fighter who keeps a Vietnamese corpse as a souvenir.
In a tense battle, Touchdown is killed by an enemy grenade, forcing Crazy Earl to become squad leader. The men secure the area, and later bury Touchdown, along with another soldier who was killed named Handjob. Cowboy explains that Handjob was also a "Section 8" due to a compulsive masturbation problem. A news crew interviews the men about the war, and they give various accounts of their experiences and philosophies in combat. With the battle over, they enjoy some downtime, and solicit a Vietnamese prostitute.
The men then cross a nearby river to where they suspect the enemy has retreated. Crazy Earl is killed by a booby-trapped stuffed animal he finds in a ruined building, leaving Cowboy as squad leader. Another soldier named Eightball tells Cowboy they are lost, and scouts a new route. When Eightball is hit by a sniper, Cowboy advises that they pull back. Another soldier named Doc Jay runs out to help Eightball, but is also shot. Although Cowboy insists that they retreat, fearing a large enemy presence, Animal Mother rushes into the battlefield and confirms there is only one sniper.
Cowboy reluctantly orders the rest of the squad to move forward, and is killed by the sniper when they pause behind a building. Yearning for revenge, the men continue forward, until Joker finds the sniper on the second floor of a burning building. When his rifle locks, Rafterman shoots and incapacitates the female sniper. The surviving squad members debate whether they should kill her, with Animal Mother wanting her to die slowly, and Joker preferring to end her pain. Joker kills her, and the men march back to the base chanting the Mickey Mouse March.
