During the Vietnam War, a group of recruits arrive at the United States Marine Corps training facility at Parris Island. Drill instructor Gunnery Sergeant Hartman uses harsh methods to train them for combat. Among the recruits is the overweight and dim-witted Leonard Lawrence, whom Hartman nicknames "Gomer Pyle," and the wisecracking J.T. Davis, who receives the name "Joker" after interrupting Hartman's introductory speech with an impression of John Wayne.
During boot camp, Hartman names Joker as squad leader and puts him in charge of helping Pyle improve. One evening while doing a hygiene inspection, Hartman notices that Pyle's footlocker is unlocked. As he inspects it for signs of theft, he discovers a jelly donut inside, blames the platoon for Pyle's infractions and adopts a collective punishment policy by which any infraction committed by Pyle will earn a punishment for everyone else in the platoon. The next night, the recruits haze Pyle with a blanket party in which Joker reluctantly participates. Following this, Pyle appears to reinvent himself as a model recruit, showing particular expertise in marksmanship. This pleases Hartman but worries Joker, who believes Pyle may be suffering a mental breakdown after seeing Pyle talking to his rifle. The recruits graduate, but the night before they leave Parris Island, Joker, who is on fire watch duty, discovers Pyle in the barracks latrine loading his service rifle with live ammunition, executing drill commands, and loudly reciting the Rifleman's Creed. Hartman is awoken by the commotion and attempts to intervene, but Pyle shoots and kills him before committing suicide, leaving Joker horrified.
By January 1968, Joker is a sergeant and is based in Da Nang for the newspaper Stars and Stripes alongside his colleague Private First Class “Rafterman”, a combat photographer. The Tet Offensive begins and the base is attacked, but holds. The following morning, Joker and Rafterman are sent to Phu Bai, where Joker searches for and reunites with Sergeant "Cowboy," a friend he met at Parris Island. However, platoon leader lieutenant Walter J. "Touchdown" Schinoski is killed by two NVA snipers, who are eliminated soon after. During the Battle of Huế, a booby trap kills the squad leader, Sgt. Crazy Earl, leaving Cowboy in command. Becoming lost in the city, the squad is ambushed by a Viet Cong sniper who kills two members. As the squad approaches the sniper's location, Cowboy is killed.
Assuming command, squad machine gunner "Animal Mother" leads an attack on the sniper. Joker locates her first, but his M16 rifle jams, alerting the sniper to his presence. As the sniper opens fire, she is revealed to be a teenage girl. Rafterman shoots and mortally wounds her. As the squad converges on the sniper, she begs for death, leading to an argument over whether to kill her or leave her to die in pain. Animal Mother agrees to a mercy killing but only if Joker will handle it, and after some hesitation, Joker shoots her. Later, as night falls, the Marines return to camp singing the "Mickey Mouse March." A narration of Joker's thoughts conveys that, despite being "in a world of shit," he is glad to be alive and no longer afraid.
