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In Africa, a European hunter named Slade, the arch-nemesis of jungle man Tarzan, arranges to steal diamonds from a defunct African mine with his crew, consisting of German diamond expert Kruger, arrogant Scot O’Bannion, boat captain Dino and the alluring Toni.  Disguised as natives, the group raids several local villages for explosives to detonate the mine shaft.  When a kindly doctor is killed during a raid, the radioman manages to transmit Slade’s name before dying, and during the burial ceremony, Tarzan determines to stop the raider.

When a visiting American adventurer-socialite, Angie, who pilots her own plane, admires Tarzan’s physique and reacts flippantly to the violence Slade has wrought, Tarzan calls her a fool.  Slade and his crew have escaped down the river on a boat, on which they are now squabbling as they head toward the diamond mine, miles away.  During a romp with Toni, Slade drops a cigarette butt in the jungle, inadvertently revealing his location to Tarzan who begins tracking them.

As Tarzan travels down river, Angie follows in her plane, but soon crashes nearby, forcing Tarzan to rescue her from a crocodile and take her along in his rowboat.  Because Tarzan and Slade are both experts on the jungle and tracking, they are soon aware of each other’s every move, and when Tarzan grows close, Slade and O’Bannion enter the jungle to attack him.  Tarzan has Angie throw a rock to mislead them, then doubles around to take aim, with his bow and arrow, at O’Bannion.  Just then, however, Tarzan turns to brush off a tarantula, revealing his position.  As Slade unmoors Tarzan’s rowboat, Slade shoots, but Tarzan manages to flee up a tree and the Europeans return to their ship.

Tarzan and Angie now must walk through the jungle, despite her exhaustion.  Finally, he stops to fashion more appropriate walking shoes for her, and though she is touched by his concern, he points out that he needs her to keep pace.  As they walk, Tarzan reveals that years earlier Slade, while hunting a rogue elephant, caught and destroyed the elephant at the expense of his entire hunting party, whom he allowed to be killed.

Meanwhile, Slade’s crew continues to bicker, with greedy Kruger secure in the knowledge that Slade cannot kill him, as only he knows how to mine the diamonds safely.  Their steering mechanism soon breaks, and as Dino works to fix it, the pugnacious O’Bannion steals his locket and takes off into the jungle, hoping to provoke him into a fight.  Dino, enraged, follows O’Bannion but soon falls into a pit of quicksand and dies.  Slade then dispassionately informs his henchman that the locket contained a picture of Dino’s mother.

Soon, Tarzan and Angie come upon Dino’s remains, and finally Angie admits that their "adventure" has serious consequences.  During their overnight trek to intercept Slade’s boat around the next river bend, Tarzan interrupts Angie’s monologue about her various social triumphs to swing her, via a vine, over the crocodiles in the bog.  When she grows too tired to go on, he carries her to a safe spot and promises to return for her.

In the boat, Toni is furious with Slade for allowing Dino to die and not showing her enough affection, and Kruger takes the opportunity to try to turn her against Slade.  The German tells her that Slade will leave her, prompting Toni to inform him that Slade loves her enough to have entrusted her with the location of the diamond mine.

Soon after, Tarzan fells two huge branches over the river, trapping Slade’s boat.  Slade throws an explosive into the jungle, wounding Tarzan, then sends O’Bannion to finish him off.  In the boat, Kruger, knowing Toni can lead him to the mine, locks Toni below deck and attempts to kill Slade by throwing explosives at him.

Meanwhile, O’Bannion finds Tarzan and, in the ensuing shootout, Tarzan manages to slay him.  When Slade sees Kruger taking off he races back to the boat, and as he starts to strangle the German, suddenly remembers that he is compelled to keep the man alive.  After the injured Tarzan fights off some unfriendly natives, Angie finds him and, determining that he needs medication, sneaks onto Slade’s boat to secure the needed supplies.  She is quickly captured by Slade, who takes her with him on foot to the nearby mine.  Though weak, Tarzan trails them to the site where they camp for the night.

While Slade searches for Tarzan, Kruger frees Angie, hoping she will find Tarzan and lead him to Slade. Then Angie locates an exhausted Tarzan, and Toni is chased by a lion.  When Slade attempts to rescue her, she falls into a trap and dies.  Slade and Kruger then go on to the mine, where they detonate the entrance.  Upon sighting the diamonds, Kruger’s avarice grows and he determines again to murder Slade.  Slade, meanwhile, ignores the diamonds in his obsession to kill Tarzan, and when he turns his attention to forming a wire noose, Kruger throws him down a shaft.  The noose catches on a crag, however, and Slade pulls himself out of the shaft and strangles Kruger.

In the jungle, Tarzan sends Angie back to the village, and although she asks him to go with her, he replies that he cannot leave his home.  Afterward, he and Slade circle each other in the undergrowth until Tarzan scales a cliff to sneak up on Slade.  Slade nearly traps Tarzan several times, but the jungle man finally overcomes the hunter and throws him off the cliff.  Bellowing his trademark yell, Tarzan returns to his home.