In 1967, U.S. Army volunteer Chris Taylor arrives in South Vietnam and is assigned to an infantry platoon of the 25th Infantry Division near the Cambodian border. Though the platoon is officially under the command of the young and inexperienced Lieutenant Wolfe, the soldiers instead defer to two of his older and more experienced subordinates: the cynical Staff Sergeant Barnes, and the more compassionate Sergeant Elias.
Taylor is deployed with Barnes, Elias and other experienced soldiers for a night ambush on a North Vietnamese Army force. An NVA patrol appears, leading to a brief firefight. Taylor is wounded while another new recruit, Gardner, is killed. Upon his return to base from the aid station, Taylor bonds with Elias and his circle of marijuana smokers while remaining distant from Barnes and his more hard-edged followers.
During a subsequent patrol, two soldiers are killed by a booby trap and another by unseen assailants. Already on edge, the platoon is further angered when they discover an enemy supply cache in a nearby village. Barnes aggressively interrogates the village chief about whether the villagers have been aiding the NVA, and kills his wife when she confronts him. He then demands information from the chief while holding a gun to the head of the man's young daughter. Elias arrives and, disgusted with Barnes, attacks him. Their fistfight is broken up by Wolfe, who orders the supplies destroyed and the village razed. Elias demands to know why Wolfe didn't act, and Wolfe plays dumb. Taylor later prevents two girls from being gang-raped by some of Barnes' men.
When the platoon returns to base, company commander Captain Harris warns he will pursue a court-martial if he finds out an illegal killing occurred, and Elias makes it clear he will testify against Barnes if given the opportunity. On their next patrol, the platoon is ambushed and pinned down in a firefight, and the situation is worsened when Wolfe accidentally directs an artillery strike onto his own unit before Barnes calls it off. Elias takes Taylor, Rhah and Crawford to intercept flanking enemy troops, while Barnes orders the rest of the platoon to retreat and goes back into the jungle to find Elias' group. Barnes finds Elias alone and shoots him, then tells the others that Elias is dead. While the platoon is extracting via helicopter, they see a mortally wounded Elias emerge from the jungle being chased by NVA soldiers, who eventually kill him. Taylor notes Barnes's cold, passive reaction to Elias's death, which horrifies the others, and realizes he was responsible.
Back at base, Taylor attempts to talk his group into fragging Barnes in retaliation when Barnes, having overheard them, enters the room and mocks them. Taylor then assaults Barnes but is quickly overpowered, and Barnes cuts Taylor near his eye with a push dagger before departing.
The platoon is sent back to the front line to maintain defensive positions, where Taylor shares a foxhole with another soldier named Francis. That night, a major NVA assault occurs, and the defensive lines are broken. During the battle, Taylor and Francis avoid a RPG blowing up their foxhole, find their courage, and kill many enemy soldiers. Much of the platoon, including Wolfe and most of Barnes' followers, are killed in the ensuing battle, while an NVA sapper destroys the battalion headquarters in a suicide attack. Now in command, Captain Harris orders air support to expend all remaining ordnance inside the perimeter. In the chaos, Taylor encounters Barnes, who has been seriously wounded. Just as Barnes is about to kill Taylor, both men are knocked unconscious by an air strike.
Taylor regains consciousness the following morning, picks up an enemy rifle, and finds Barnes slowly crawling along the ground. Barnes orders him to call a medic, but Taylor does not respond. After daring Taylor to kill him, Barnes is shot dead by him. Francis, who survived the battle unharmed, deliberately stabs himself in the leg and reminds Taylor that because they have been twice wounded, they can return home. As a helicopter carries the two men away, Taylor waves goodbye to Rhah. Sgt. O'Neil, a Barnes lackey who hid under a dead body during the battle, is given command of the rest of the platoon. Overwhelmed, Taylor breaks down sobbing as he flies over multiple craters full of corpses, narrating how the war has changed him forever.
