In South America, Americans Steve Bailey and Marty Lang, who have been hired by a petroleum company, are planning an expedition into dangerous Motilone Indian country to look for oil. The company's head, Francisco Servente, warns them about the dangers of "Jaguarmen," murderous, jaguar-worshiping Motilone Indian priests identified by the jaguar skins they wear and the smell of the mind-altering drug, "chica," of which they partake. Servente advises the Americans to enlist the help of ethnologist Dr. Henry Powell, who has led many jungle expeditions, and suggests that they pretend they are hunting jaguar, rather than searching for oil, so as not to interest "cutthroat" rivals. While conferring with Powell, Bailey and Lang meet his adopted son Juano, a Motilone whom the doctor rescued from the jungle fifteen years ago. Juano is offended when the Americans treat him as a curiosity and the middle-aged Bailey makes unwelcome advances toward Juano's girl friend Rita, who is also Powell's secretary. Although he remembers little of his jungle experience, Juano, already self-conscious about his "savage" background, is troubled by their rudeness. Juano wants to accompany Powell on the expedition, but his father refuses, as it will interfere with his plans to begin college. However, when Juano learns that his admission has been postponed by the college due to lack of space, he bitterly assumes that the postponement was based on his race and vows to return to the jungle. While the riverboat is being loaded for the expedition, Juano and Powell witness the Americans, who are secretive about their equipment, mistreating the local workers hired as porters. When a porter gets too close to the equipment, Bailey assaults him, pushing him into the river where he is eaten by piranhas. Tupi, a longtime friend of Juano's hired to help with the expedition, confides that the Americans' equipment is for collecting oil, not jaguars, as the Powells have been told. Fifty miles from Motilone country, Powell instructs the men to build a stockade. That night, a group of Jaguarmen shoot a Motilone arrow at the stockade's gate. Hearing the Indians signal each other with bird sounds, Juano is surprised to find that he can understand them. Back in town, meanwhile, Rita receives a letter stating that the college now has an opening for Juano in the coming school term. She hires Garcia Solimos, who owns a motor boat, to take her to rendezvous with the expedition so that she can tell Juano the news. Upon learning of his acceptance by the university, Juano, disturbed by his familiarity with the Motilone calls, confides to Rita that he feels compelled to prove to himself that the jungle is not his home. Before Rita and Garcia can return to town, Motilones set both boats adrift and leave the sign of the claw mark on the stockade wall, thus causing fear among the porters. Juano, troubled, tells Powell that he somehow knows that the mark, called "sukuruju," means "death." Powell suggests that Juano may have been present at a tribal ritual years ago and retains the knowledge subconsciously, but Juano concludes that "the jungle is still part of me." During the night, Tupi asks Juano for his protection from the Jaguarmen, believing that a claw-shaped tattoo Juano received as a child connects him to them. After Juano single-handedly kills a cornered jaguar that threatens the group, its body is brought to camp. When its head and feet are later found mysteriously cut off, the men wonder if Juano was responsible, as no other Motilone entered the stockade. Later, as the expedition enters the oil-rich valley in which the Motilone nation resides, Powell, aware of the Americans' true goal, insists that they make friends with the hostile Indians. Fearing that Powell will divulge the existence of oil and thus instigate a rush of fortune hunters, Bailey and Lang plan to desert the expedition as soon as they finish their tests so that they can return to town and file a claim for themselves rather than Servente's company. The group gives refuge to an injured Campa Indian who was poisoned by a dart from a Motilone blowgun, but during the night the Campa is killed by a man dressed in Jaguar skins who is fleetingly glimpsed by members of their group. Afterward, Lang points out that there is blood on Juano's sleeve, casting suspicion on Juano for the murder. Another night, Bailey, drunk, drags Rita from her bed and tries to kiss her, and Juano almost kills him. When Bailey is found dead the next morning, clawed to death, Lang immediately directs suspicion toward Juano, and Tupi's discovery of chica in Juano's bedding seems to confirm the young man's guilt. However, Juano claims to have no memory of taking the drug or of killing Bailey. When Juano asks Powell if it is possible to kill without remembering, the doctor says that it is, when under the influence of chica, but believes there is another explanation. At Powell's direction, Juano is tied to his hammock the next evening, so that he cannot wander during the night. Powell writes in his diary that he has pretended to knows who the real killer is in an attempt to trick the person into revealing himself. However, Powell is killed by a man in jaguar skins during the night, after which Juano, reeking of chica, is found untied. Fearing that he is a murderer, Juano despairs that killing is in his blood and runs off into the jungle. Powell's death prompts the rest of the workmen to flee, except for Tupi, who forces Lang to make him a "partner." Alone in the jungle, Juano is attacked by an anaconda, but saved by Motilone, who take him prisoner. After dressing him in a jaguar skin and giving him chica, they present him with a boar that he is expected to kill. However, when he is unable to make himself kill it, they prepare to ritually murder him. The appearance of a jaguar scares them off, allowing Juano time to free himself and slay the jaguar. Once he is reunited with Rita, who has followed him into the jungle, he tells her that his recent experiences made him realize that he has killed no man and that savagery does not run in his veins. As Rita and Juano race through the jungle toward home, the Jaguarmen follow their tracks, but rain washes away their footprints, enabling their escape. Upon reaching the river's edge, they see a man in jaguar skins murdering Tupi. Juano attacks the man and, after overpowering him, pulls of his jaguar skins and discovers that the killer is Lang, who has been impersonating a Jaguarman. Lang tries to escape in a raft, but Juano swims out and, after a struggle, knocks him into the water, where he is eaten by piranhas. Juano and Rita then raft toward home.