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African guide and animal expert Jungle Jim discovers several animals acting strangely lethargic and summons the game warden and doctor to investigate. The doctor requests several animal specimens and Jim and his pet chimp Tamba go in search of a baby elephant that Jim had noticed behaving oddly. Unknown to Jim, corrupt scientist Dr. Andrews and a large expedition are conducting a series of experiments with an ore unique to the area that is distilled down to an extract, which when drunk, reduces the subject's will to resist. Andrews and his men test the extract on the jungle animals and, after following an elephant herd, take the baby for further study. Nearby, Tamba is nearly captured by a Wasuli native woman, Shari, until Jim comes to the rescue. Shari attacks Jim, but their struggle is broken up by her fiancé, Ramada. Shari's brother, Mahara, the Wasuli chief, explain that their wandering tribe follows migrating animals and captures them to sell to zoos and circuses. Jim warns them that they are near the Canyon of the Ape, home of a giant ape-man who kills all who enter his territory, but Ramada and Shari remain doubtful. After leaving the Wasuli, Jim follows Andrews' men and the baby elephant toward their camp in the Canyon of the Ape, as the ape-man observes them. When the ape-man tries to attack Tamba, Jim fights him off with a knife, but has little effect. Jim and the chimp finally are forced to outrun the slow, lumbering giant. At Andrews' camp, Jim advises the men to relocate as they are in the middle of the dangerous canyon, but Andrews is skeptical. Before departing, Jim expresses concern over the caged animals, who appear nearly comatose. When an antidote against the extract fails to revive the unconscious animals, Andrews orders his henchmen to find the Wasuli and buy all their animals. Suspicious of Andrews' true motives, Jim has remained nearby and overhears the orders. He then follows Andrews to nearby caves where he observes several more caged animals and an elaborate laboratory. The ape-man also follows and enters the caves, disrupting the lab and cages. Jim escapes, but the ape-man follows as Jim hurries after Andrews' men to prevent the animal sale. Andrews and his men meet with Mahara and assure him that there have been no sightings of any ape-man, and Shari insists Jim has lied to them. Later at the Wasuli camp, Mahara orders Ramada to deliver the animals to Andrews' camp while he goes in search of Jim to verify whether he is their friend or enemy. Just outside his camp, Mahara is attacked by the ape-man. Jim arrives and tries to save him, and in the ensuing struggle, Mahara is hurled upon Jim's knife. As the ape-man flees, Shari, Ramada and several Wasuli discover their dead chief and accuse Jim of his murder. The Wasuli take Jim back to their camp, but disbelieve his story of the attack. As aging wise man Achmed advises Shari to reconsider, Tamba begins freeing all the caged animals, allowing Jim to escape in the confusion. Shari follows and finds him on the trail, but Jim overpowers her and takes her back to his camp to convince her of the danger of the ape-man. That night, the ape-man attacks before Jim drives him off with a burning branch. Shari apologizes for doubting Jim and promises to convince Ramada of the truth. Meanwhile, Ramada has rounded up the animals and sets off with Achmed to Andrews' camp, just missing Shari and Jim's return. Jim follows Ramada, but soon after his departure, the ape-man attacks the Wasuli village and carries off Shari. Ramada and Achmed arrive at Andrews' camp, but when they ask about the use of the animals and the scientist refuses to answer, they withdraw from the sale. The men attack the Wasuli and Ramada is subdued. Later, Jim slips into the camp and frees Ramada, just as the ape-man arrives carrying a shrieking Shari. Andrews' men overcome the ape-man and stake him to the ground, where they try to force him to drink the extract without success. To get them to drink the extract, Andrews also withholds water from the captured Jim, Ramada and Shari, but they refuse for three days. Tamba then leads the jungle monkeys in an attack against the camp and during the ensuing melee, frees Jim and the others, while the ape-man also breaks free. When Andrews and his men realize the ape-man has escaped, they begin packing hastily, but the giant returns to the caves and kills them all. Determined to save the caged animals, Jim and Ramada go into the caves where, after a long struggle, Jim traps the ape-man in a fire. Upon his report to the warden of the affair, Jim is promised a medal for his heroic actions.