Lights, camera, action!
It's 1967, and a plane full of new soldiers has just arrived in Vietnam. Among them is Chris Taylor (Charlie Sheen), who's about as green as they come. He and the other newbies pass by a group of battle-hardened veterans before being shepherded elsewhere. Early in his tour, Taylor is running point (the guy who goes first in a patrol) and passes out. Elias (Willem Dafoe) helps him recuperate, telling him that he's carrying too much junk in his pack for the kind of patrol they're doing.
Sometime after this first romp through the jungle, Barnes (Tom Berenger), Elias, Wolfe (Mark Moses), and a few other sergeants are convening: Elias' squad must go on night patrol. O'Neill (John C. McGinley) thinks he's won a victory but he's ordered to go as well. During the night ambush patrol, Taylor stands guard. After the end of his shift, he wakes Junior, who falls asleep instead of manning his post. Taylor wakes up later, notices Junior isn't paying attention, and watches NVA troops approach. A firefight ensues, one guy is killed, and Taylor is unable at first to use his Claymore because he's failed to turn the safety off. He is wounded and thinks he's dying, but it's only a flesh wound. Barnes warns them about falling asleep on watch.
A few days later, Taylor returns to camp, a bandage on his neck, and works latrine duty with King (Keith David) and Crawford (Chris Pedersen) before heading into the Underground to smoke pot and get drunk. Elsewhere, Barnes, O'Neill, Bunny (Kevin Dillon), and others hang out in the barracks (they are not part of the "pothead" group).
A few months later (New Year's Day, 1968), the platoon goes on patrol again, this time during the day. Taylor is running point and calls a halt because he senses something. Barnes notices it too, and points out that it's a bunker. NVA troops have just left the area, and the platoon fans out to inspect the surroundings. Elias investigates the interior of the bunker complex. Outside, two guys inspecting a cache of maps are blown up. There are booby traps all over the place. Meanwhile, Manny (Corkey Ford) has wandered off and is nowhere to be found.
Soon, the platoon finds Manny—mutilated and tied to a tree. Barnes is enraged, as are the others. The platoon enters a nearby village, discovers a cache of weapons, and wreaks havoc. Bunny kills a one-legged villager, and Barnes kills the village chief's wife. He's about to kill the village chief's daughter before Elias stops him and attacks him. Wolfe, who is nominally in charge, pretends not to have seen anything. The soldiers burn the village and leave.
Back at camp, Wolfe and Elias are speaking with Captain Harris (Dale Dye). Elias plans to file a report. Harris warns them that there will be a court martial if there was an illegal killing. The platoon is now divided, a fact clearly evident in the support and comments Barnes receives from O'Neill, Bunny, and others.
The next day, the platoon is sent back to the bunker complex to investigate further. They are quickly ambushed, and a chaotic firefight ensues. The incompetent Lieutenant Wolfe calls in the wrong coordinates for an air strike, some guys almost bite the dust, and Barnes is livid. Elias understands how the NVA army plans to crush them, and he takes a small group of guys to try a flanking maneuver. The group halts the enemy advance, and returns to the rear but without Elias.
Barnes, sensing an opportunity, goes back into the jungle, and eventually runs into Elias, whom he murders—or so he thinks. Meanwhile, Taylor, a bit anxious about Elias, goes back into the jungle looking for him. He runs into Barnes, who tells him Elias is dead and that he needs to get back to the rear. Taylor is suspicious. As the troops are being evacuated, Elias emerges from the trees, wounded, but not dead. Barnes watches Elias try to escape with unusual interest. Taylor notices, and realizes that Barnes had killed Elias. Barnes, however, has nothing to worry about as Elias is quickly dispatched by NVA troops.
Back at camp, Taylor is in the underground (nicknamed the "Underworld") drug den with Rhah (Francesco Quinn) and others. He says he knows Barnes killed Elias, and suggests they "frag" him. The others are skeptical, especially Rhah, for whom Barnes is a strange, immortal anomaly ("Barnes ain't meant to die"). Barnes comes in, having overheard some of their conversation, and gives them a lecture about what's necessary for the smooth running of a military machine. He knows he's in hostile territory, and he dares the guys to kill him. At first nobody responds, but when he's about to leave Taylor attacks him. Barnes gains the upper hand, pulls a knife, and seems ready to kill him. Rhah talks him out of it, and Barnes just cuts Taylor's eye.
Shortly thereafter, the men return to the area where Elias was shot. They suspect that an entire NVA division is nearby, and they know a big fight is coming. The men prepare their foxholes, and Taylor suggests to King that he's made a big mistake.
Later that night, NVA troops attack. Taylor is able to fend some of them off, but eventually must abandon his foxhole before it's blown up. Eventually, the entire perimeter is overrun, and many of the platoon's members are killed in the chaos (Wolfe, Bunny, and Junior are among them). O'Neill hides under some dead bodies to try to save his own skin.
Barnes is annihilating enemy soldiers in hand-to-hand combat when Taylor finds him. Barnes tries to kill Taylor, but an air strike knocks them both unconscious. The following morning, Taylor stumbles over to where Barnes an almost killed him, grabbing an enemy weapon on the way. Barnes dares him to shoot him ("Do it," he says), and Taylor obliges. Short thereafter, Taylor is put on a helicopter and sent home (this being his second wound), a young man forever changed and forever haunted by the war.