In September of 1967, Chris Taylor enters the Vietnam War after dropping out of college. He joins an infantry stationed near the Cambodian border, and struggles to acclimate to the physical demands of soldiering in the jungle. When Taylor vomits on a patrol after seeing the corpse of a Vietnamese soldier, Sergeant Bob Barnes reprimands him, but Sergeant Elias later comes to his aid and lightens the load in his pack. In a letter to his grandmother, Taylor describes his duties, including walking all day, digging foxholes in the evening, and going on all-night ambushes, and worries that he has made a mistake by volunteering for the war. One afternoon, Barnes orders Elias to lead his men on a night patrol, but Elias argues that it is Sergeant O’Neill’s turn. Lieutenant Wolfe, the platoon commander, believes he should be the one giving orders and confronts Barnes, who agrees begrudgingly. In the jungle late at night, Taylor wakes up fellow soldier, Junior, when it is his turn to stand watch; however, Junior quickly falls back asleep, and Taylor later wakes to find soldiers from the North Vietnamese Army (NVA) circling their camp. A firefight ensues, and Taylor is grazed by a bullet while another new recruit, Gardner, is killed. After Taylor returns from a short hospital stay, King, a fellow soldier, invites him to party with Elias and a group of soldiers called “hopheads,” who use drugs, listen to rock n’ roll music, and dance together in their bunker. While Taylor bonds with Elias’s crew, a more aggressive group of soldiers drinks beer and listens to country music while Barnes and O’Neill play poker in their bunker. On New Year’s Day, 1968, the platoon patrols the jungle and comes across a bunker recently abandoned by the NVA. Two men are killed by an explosive booby trap, while another soldier, Manny, goes missing. After the platoon finds Manny’s mutilated body tied to a tree by the river, Wolfe receives orders for the platoon to search a nearby village where Vietnamese soldiers may be hiding. The platoon rounds up villagers, mostly women and children. When Taylor finds an old woman and a mentally handicapped, one-legged man inside a hut, he shoots at the man’s foot, forcing him to hop around. As Taylor walks away, Bunny, one of Barnes’ men, laughs and continues the abuse, brutally bludgeoning the handicapped man until he dies. Outside, Barnes interrogates the village chief, who says the villagers were forced to stash weapons but he doesn’t know when the army will be back. The village chief’s wife demands to know why Barnes’s men killed their livestock, and Lerner, a soldier who speaks Vietnamese, translates, but Barnes becomes agitated and shoots her in the head. He then holds the village chief’s daughter at gunpoint, demanding more information; however, Elias arrives and hits Barnes with the butt of his rifle, ordering him to stop. The sergeants fight, but Wolfe interferes. Elias demands to know why Wolfe allowed Barnes to shoot the woman, but Wolfe feigns ignorance. In the bushes nearby, Taylor comes across a group of soldiers, including Bunny and Junior, who watch while their companion, Morehouse, rapes a young Vietnamese girl. Taylor pulls Morehouse off the girl and yells at the group, calling them animals. His comrades ridicule him, and Bunny calls Taylor “a homosexual” for interrupting. On Wolfe’s orders, the platoon sets fire to the village and leaves. Later that day, Elias informs Captain Harris that Barnes shot the village chief’s wife. Harris orders Elias and Barnes to make a report when they return to base camp, and promises there will be a court-martial if he finds that Barnes’s actions were illegal. That night Elias tells Taylor that he no longer believes the United States can win the war. On the platoon's next patrol, they are ambushed by NVA soldiers. Elias devises a plan to strike the NVA from behind, and takes four soldiers, including Taylor and Rhah, with him as he sets off to another location. Meanwhile, Wolfe provides the wrong coordinates for an air strike, and many of his men are killed. After he reprimands Wolfe, Barnes takes charge and orders everyone to retreat to helicopters. Barnes then searches for Elias’s group. When he finds Elias separated from the others, Barnes shoots him down. Moments later, Barnes runs into Taylor and tells him that Elias was killed in action. Though Taylor wants to find Elias, Barnes orders him to escape in the helicopters. As the surviving soldiers are flown out of the area, they spot Elias on the ground below, running from a group of NVA soldiers. As Elias is shot several more times and falls to the ground, Taylor glares at Barnes, realizing his previous report of Elias's death was a lie. Back at the base, Barnes hears Taylor secretly urging the other soldiers to kill their sergeant, but no one moves when Barnes dares them to try. Taylor attacks Barnes, but the sergeant overpowers him and holds a knife to his face. Although Rhah urges Barnes to back off, Barnes makes a cut under Taylor’s eye before leaving. Wolfe commands Rhah to head Elias’s squad, but Rhah argues that the squad has only six people remaining. O’Neill, who has reservations about the platoon’s upcoming mission, asks Barnes if he can take his rest and recuperation leave early, but Barnes rejects the request, claiming that everybody must die sometime. That night, Taylor and his companion, Francis, watch from a foxhole as NVA soldiers invade their camp. Though they are outnumbered, Francis and Taylor gun down several soldiers and escape the foxhole just before it is hit by a grenade. Taylor runs through the camp, on a killing spree, shooting one soldier after another. Meanwhile, Harris orders the Air Force to “expend all remaining” within the surrounding perimeter. Barnes and Taylor cross paths on the battlefield, and though Barnes attempts to kill Taylor with a shovel, the two are knocked out by an air strike. Taylor wakes up the next day, surrounded by corpses. He sees Barnes crawling through the jungle, severely injured, and shoots him dead. A search crew later finds Taylor sitting on a log, holding a grenade. Francis emerges from a foxhole and stabs himself with a bayonet in order to be taken away as a casualty. O’Neill emerges from his hiding place under the body of a dead NVA soldier. To O’Neill’s dismay, Harris gives him control over the platoon when gets back to camp. Francis informs Taylor that they get to go home since they've been wounded twice. As Taylor is lifted away by helicopter, he waves goodbye to Rhah, then holds himself and weeps.