In 1964, Major Asa Barker leads a unit of American military advisors stationed in Penang, South Vietnam. When a group of replacement troops arrive, Barker meets with them individually. They include the incompetent yet blindly optimistic Southerner, Lieutenant Raymond Hamilton; war-hardened, alcoholic Sergeant “Oleo” Oleonowski, who served in Korea under Major Barker; Corporal Abraham Lincoln, an opium addict; and the compassionate yet inexperienced Corporal Courcey. Barker’s commanding officer, General Harnitz, orders Barker to reoccupy and garrison an abandoned village called Muc Wa, but Barker disapproves of the mission, telling his executive officer, Captain Alfred “Al” Olivetti, that by attempting to occupy numerous strategically unimportant sites, the U.S. will repeat the mistakes of the French in the First Indochina War. However, Barker proceeds warily with the assignment, naming Hamilton the commanding officer at Muc Wa in order to spare the burnt-out Oleo from too much responsibility. Also assigned to the Muc Wa unit are the remaining new recruits, Lincoln and Courcey, as well as Sergeant “Cowboy” Nguyen, a merciless Vietnamese soldier who is highly skilled as a translator, and a group of South Vietnamese soldiers. Before leaving, Oleo airs his misgivings about the mission, telling Barker that the South Vietnamese troops are no more than poorly trained farmers and that Muc Wa will be an easy target for their enemy, the Vietcong. Barker is unconvincing in reassuring Oleo. While Barker and Olivetti stay behind in Penang, the Muc Wa unit sets out on foot. Although Hamilton has been placed in charge, Oleo emerges as the natural leader, spotting an explosive trap shortly into their journey. The Vietcong soldier who set the trap attempts to escape, but is caught by the South Vietnamese troops. While the others look on, Cowboy beheads the man with a machete. The sight causes Hamilton to vomit, and when Courcey cannot stop staring, Oleo says, “It's their war.” Reaching Muc Wa, the group sets up a base and explores their surroundings. Courcey and Hamilton discover a graveyard containing over three hundred French gravestones. An inscription over the entrance reads: “Stranger, when you find us lying here, go tell the Spartans we obeyed their orders.” Courcey explains that these were the words of the Greek poet Simonides's epitaph for the three hundred Spartans who died in the Battle of Thermopylae. Hamilton later acknowledges the bravery of the French, but asserts that his group will not lose because they are American. Later, Cowboy and Courcey discover a Vietnamese family gathering food by a river. Although Cowboy insists the family is Vietcong and wants to kill them, Courcey overrules him and brings them back to the camp as refugees. Oleo reprimands Courcey, believing that the Americans should defer to Cowboy's judgment when attempting to identify Vietcong. Hamilton disagrees and welcomes the refugees to their base. Later, when spotting an elderly, one-eyed Vietcong soldier in the graveyard, Courcey does not draw his gun fast enough, and the man escapes. Courcey warns the others shortly before the Vietcong launch an attack that night, which the Americans successfully fend off. Courcey, who shot and killed some of the Vietcong attackers, is shocked to discover that one was a young girl. The enemy attacks in greater force the following night, and Hamilton is shot to death when he attempts to rescue a wounded South Vietnamese soldier. Devastated, Oleo gets drunk the next morning and kills himself. With both Hamilton and Oleo dead, Barker sends Olivetti to take charge of Muc Wa, then goes to see Colonel Minh, the province chief of the South Vietnamese Army, whom Barker secretly calls “Lardass.” Minh initially refuses Barker’s request for three hundred South Vietnamese troops, explaining that his forces are needed in Saigon to combat an expected coup; however, Barker is able to bribe him with a large supply of artillery. That night, with the three hundred reinforcements not yet arrived, Muc Wa is greatly outnumbered by Vietcong in another firefight. Olivetti requests air support, which General Harnitz refuses to provide, citing the expected coup in Saigon. Barker personally persuades Harnitz to provide the air support, which helps repel the attack. The next day, as Barker leads the South Vietnamese reinforcements to Muc Wa, he learns that 1,000 Vietcong soldiers are also on their way and Harnitz has ordered the base’s evacuation, claiming that Muc Wa is not strategically important enough to justify a such a large confrontation. Barker sends the South Vietnamese back to Penang as he boards a helicopter to Muc Wa. With the approaching attack, it becomes apparent that escape via aircraft is the only way out of Muc Wa. However, the helicopter only has room for a handful of American military advisors and the severely wounded. Although Cowboy and the other South Vietnamese are desperate to board the helicopter, Barker pushes them back. Unwilling to abandon his allies, Courcey refuses Barker's order to board, prompting Barker to stay behind as well, knowing he has joined a tragic cause. As the group at Muc Wa prepares for their evacuation on foot, Barker reveals to Courcey his belief that the U.S. involvement in Vietnam is pointless. That night, Cowboy catches the camp's Vietnamese refugees attempting to escape with guns and ammunition, and shoots them dead, confirming that they were Vietcong. Attempting an escape, Barker’s group encounters an overwhelming attack, and Cowboy is the first of many to die. When both Barker and Courcey are shot, Barker carries the younger soldier to temporary safety before both lose consciousness. Later, a South Vietnamese soldier discovers Courcey still alive, and takes him to a more secluded spot as the fighting continues. The next morning, Courcey awakens to a landscape covered with dead bodies and sees Barker’s corpse naked and face down in the mud. He stumbles into the French graveyard where he spies the one-eyed Vietcong man, wounded but still alive. The man weakly tries to aim his gun, but it drops out of his hands. Courcey announces that he is going home if they will let him, then limps out of the graveyard.