The film begins with footage of the Vietnam Veterans Memorial.
It then shifts to a platoon of soldiers fighting in Vietnam, 1969 ending with a soldier dying on a helicopter. As they prepare to be sent into action again, the platoon of the 3rd Battalion, 187th Infantry, part of the 3rd Brigade, 101st Airborne Division, receives five FNGs as replacements - Beletsky, who constantly frets that he won't be able to remember everything he has been taught; Languilli, who gets annoyed when people mispronounce his name; Washburn, a quiet man and the only African-American member of the squad; Bienstock, who is outgoing and has volunteered for combat duty in Vietnam; and finally Galvan, the quietest new soldier but the most promising of the new intake.
Taken under the wing of their war-weary squad leader, Sgt. Adam Frantz, the recruits are taken through a crash-course in battlefield skills, including everything from oral hygiene to a demonstration by a Viet Cong deserter as to how skilfully enemy troops can penetrate perimeter U.S. defenses.
The platoon has a new commander, Lieutenant Eden, who is going to need the skills and experience of both Frantz and the Platoon Sergeant, Sergeant First Class Worcester. The platoon's machine gun team is composed of the burly Private Duffy and his mismatched, bespectacled companion, Private Gaigin. There are also three African-American veterans in the unit: Motown, "Doc" Johnson and Sgt. McDaniel, all of whom have first-hand knowledge of the racial discrimination still practiced in the army.
The new arrivals get their first sudden taste of war when a quiet spell beside a river is interrupted by an enemy artillery barrage and Galvan is killed. The platoon enters the A Shau Valley and makes contact, sparking a firefight in which Sgt McDaniel is killed. This loss provokes considerable bitterness and tension as McDaniel was near the end of his tour and, being black, was denied any chance at rear-area duty at headquarters.
The battalion is initially ordered to reconnoiter a nearby mountain but is unexpectedly diverted and commences an assault on the enemy-held Hill 937 which soon grows into a major battle as unexpectedly heavy resistance is encountered and the NVA, rather than using hit-and-run tactics, are instead defending well-entrenched positions. The platoon is forced to attack the hill repeatedly against stubborn opposition and US napalm air-strikes steadily strip away all vegetation, leaving the hill a barren, scorched wasteland. In one assault, a battle-crazed Duffy, wielding an M60, seems on the verge of carrying the day as the enemy begins to crumble. But misdirected fire support by helicopter gunships causes many friendly-fire casualties, to the horror of Lt. Eden and his radio telephone operator, Murphy, who called in the airstrike. The assault fails and Duffy is among the fatalities.
In between attacks, the shrinking platoon tries to rest, chattering about social upheaval and unrest back home. Bienstock is devastated by a Dear John letter from his girlfriend, whose college friends have told her that it is immoral to remain partnered with a soldier. Beletsky gets a letter on tape from his girl back home and Frantz is surprised (and moved) that she mentions his name. Sergeant Worcester describes to his comrades the alienation and hostility he encountered on his return home from his previous tour of duty, along with the collapse of his marriage and how a good friend, whose son had been killed in Vietnam during the Battle of la Drang in 1965, had been driven to an emotional breakdown by cruel phone calls from anti-war college students gloating over his son's death. Frantz makes clear his lack of patience for draft-dodgers, and states that they should at least appear in the war effort, regardless of having no will to fight. He also has an angry confrontation with a TV reporter, telling him that he has more respect for the NVA on the hill than for the reporter because "at least they take a side".
The increasingly exhausted platoon continues attempting to capture Hill 937, but to no avail. The tenth assault takes place in torrential rains, turning the hillside into a river of mud. Gaigin is killed, Beletsky is wounded, and Doc Johnson is shot by an NVA soldier hidden in a hole. Before he is evacuated, Doc tells Frantz and Motown to capture the hill so that they will at least have something to be proud of, but succumbs to his wounds moments later. Beletsky, despite having received a "million dollar wound," decides to return to his unit.
The 11th and final assault is mounted by the remaining troops whose bitterness and exhaustion is overcome by anger and unit pride. The final enemy positions are overrun but the cost is heavy. Lieutenant Eden is seriously wounded, losing his arm. Murphy, Worcester, Motown, Bienstock and Languilli are all killed before the few remaining troops make it to the summit. Frantz, wounded by an enemy bayonet, rests on the hilltop alongside Beletsky and Washburn as the battlefield finally goes silent. The final image is the now battle-aged, haunted face of Beletsky as he gazes at the carnage below, eyes glistening with tears. Constant radio chatter is overheard, but there is no reply.
Who lost his arm?
Lieutenant Eden