A spacecraft flies near Earth and releases an object which enters the atmosphere. Some time later, in a Central American jungle,[notes 1] former U.S. Army Special Operations soldier and Vietnam Veteran Major Alan “Dutch” Schaefer [notes 2] and his elite mercenary rescue team — Mac, Billy, Blain, Poncho, and Hawkins — are tasked by Dutch’s former commanding officer, General Philips, with rescuing a foreign official and his aide held hostage by insurgents. CIA agent Dillon, a former commando and an old friend of Dutch’s from the Vietnam War, is assigned to accompany the team over Dutch’s objections.

The team discovers the wreckage of a helicopter and three skinned corpses, identified by Dutch as Green Berets out of Fort Bragg that he knew personally. Dutch’s team reaches the enemy camp and kills all of the insurgents, including a Soviet intelligence officer searching through valuable documents. Confronted by Dutch, Dillon admits the mission was a setup to retrieve intelligence from captured operatives and that the dead military unit disappeared weeks earlier in a failed rescue.

After capturing Anna, the only surviving guerrilla, the team proceeds to its extraction point, unaware that it is being tracked with thermal imaging by an unseen observer. Anna escapes and is chased by Hawkins, but they are ambushed by the creature. It spares Anna but kills Hawkins and drags his body away. Dutch organizes a search for Hawkins’ body, during which Blain is killed by the creature’s plasma weapon. Enraged, Mac initiates a firefight in which the creature is wounded, revealing luminescent green blood. The unit regroups and realizes that something in the jungle is stalking them. Dillon believes more guerrillas are responsible, but Billy is adamant that the perpetrator is not human, an assertion that is met with skepticism. The team makes camp for the night, setting traps in all directions.

That night the traps are set off, and Mac kills a wild pig, mistaking it for the creature. In the confusion, the creature steals Blain’s body and Dutch realizes that their enemy uses the trees to travel, making their traps ineffective. A second attempt to capture the creature using a net momentarily succeeds, but it quickly escapes, leaving Poncho injured. Mac and Dillon try to pursue it, but the alien outmaneuvers and kills them. The creature then catches up with the others, killing Billy and Poncho and wounding Dutch. Realizing the creature does not target unarmed prey because there is “no sport” in it, Dutch sends Anna to the helicopter unarmed. The creature pursues Dutch into a river, and its cloaking device malfunctions. The creature, now visible, goes within a few feet of a mud-covered Dutch. His thermal signature reduced, Dutch remains unseen by the creature and it moves on. Dutch realizes he can use mud as camouflage.

While the creature collects trophies from the dead mercenaries, Dutch crafts traps and weapons and lures the creature to him. He disables the creature’s cloaking device and inflicts minor injuries but falls into a river, losing his mud cover, and is pinned by the creature. Acknowledging Dutch as a worthy foe and wishing to fight him in equal terms, the creature discards its mask and plasma caster and engages him in hand-to-hand combat. Dutch is almost beaten, but manages to crush the creature under a trap’s counterweight. As the creature lies dying, it activates a self-destruct device while laughing maniacally in Billy’s voice. Dutch takes cover just before the device explodes. He is rescued by Philips and Anna in the helicopter, beaten and dispirited.