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Tarzan the Ape Man wrestles a lion about to attack Jeff Herbert, jungle guide for Mary Brooks and her sweetheart Bob, who are searching for Mary's father, Dr.  Brooks, a scientist studying African races and religions.  Jeff shows Tarzan a letter from the solicitor to the executor of the late Lord Greyfriar's estate, which offers £10,000 for proof of the rumor that Tarzan, Lord Greyfriar's son and heir, has been killed.  Tarzan, being illiterate, ignores the letter and runs off.

At Dr.  Brooks's hut, the party finds a map to a treasure chamber, which Jeff's cohort Nick tries to steal, but when Bob fights him, Tarzan crashes through the roof on a tree limb.  He then revives Mary and takes her phonograph and picture to his cave.  After Mary is abducted to be sold to the high priest of a tribe which worships Zar, an idol with large emeralds implanted in its knuckles, Tarzan rescues her and brings her to his cave.  There she plays the phonograph for him.  Meanwhile, Nick is killed by a lion after he steals the map.

After Jeff and Bob are captured and brought to a cave, where the high priest is about to sacrifice Dr. Brooks, Tarzan rescues them, but Jeff steals one of the idol's emeralds.  At Tarzan's cave, Jeff, by threatening to shoot Tarzan for the reward, exacts a promise from Mary that she will marry him.  While swinging on vines with Mary, Tarzan falls into a pit, and he is about to be attacked by a lion, when an elephant, alerted by an ape, scares the lion away and pulls Tarzan out.

When the tribesmen capture Mary, the emerald is replaced, and Bob trades the map to the high priest for their freedom.  When Jeff tries to shoot Tarzan, they struggle and the gun goes off killing Jeff.  Before he dies, he apologizes to Mary and gives her the letter about Tarzan.  Mary, who worries that Tarzan, who has disappeared, is badly hurt, is overjoyed to find that he has moved into her father's hut.  As she tries to teach him English, Tarzan's chimpanzee friend dances to a phonograph record, as do an elephant and some other apes.