A shipping disaster in the nineteenth century has stranded a man and woman in the wilds of Africa. The lady is pregnant, and gives birth to a son in their tree house. The mother dies soon afterwards. An ape enters the house and kills the father, and a female ape takes the tiny boy as a replacement for her own dead infant, and raises him as her son. Twenty years later, Captaine Phillippe D'Arnot discovers the man who thinks he is an ape. Evidence in the tree house leads him to believe that he is the direct descendant of the Earl of Greystoke, and thus takes it upon himself to return the man to civilization.
- Scotland, 1885. The young John Clayton of the house of Greystoke (Paul Geoffrey) plans a voyage to Africa, to one of the colonies near the coast, along with his pregnant wife, Alice (Cheryl Campbell). John bids farewell to his father, the Sixth Earl of Greystoke (Ralph Richardson).
Months later, off the coast of Equatorial West Africa, we see the remnants of a shipwreck, the very one chartered by Clayton. He and his wife have survived the wreck, but are now marooned in the jungle. They set up a house in the trees using timber and decking from the ship. Alice soon gives birth to their baby, a son named Jonathan Jr., but develops a severe fever soon after. During her sickness, Clayton writes in his journal of the difficulties in caring for his ill wife and their baby. Alice's breast milk becomes virulent and she deteriorates further. One day, Clayton checks on his wife to find her dead. As he weeps over her body, a large primate called White Eyes, leader of a nearby clan of apes, enters the home. Petrified, Clayton lunges for his gun but is quickly overtaken by the ape and killed when the ape charges him and beats him to death. Several other apes from the clan enter the house and begin to curiously inspect the house. A female named Kala, mate of the ape Silverbeard, enters the home carrying the body of her recently deceased baby. When she discovers John Jr. crying in his crib, she drops her baby's lifeless body and claims John as her own. She flees the house with the rest of the apes as one of them accidentally discharges Clayton's revolver while examining it.
John (at age five Danny Potts) grows, brought up as an ape with the assumed name of Tarzan, but falls under constant persecution from White Eyes, who considers him an outsider because of his hairless skin. Despite trying hard to fit in, Tarzan's human attributes prove beneficial to him. Sipping water from a stream with a peer, Droopy Ears, Tarzan is forced into the water when the two are attacked by a leopard. Tarzan's friend is killed but he finds that he has the ability to swim to safety, whereas the apes would have normally stayed out of the water.
Later (at age 12 Eric Langlois), Tarzan eventually discovers the home his human mother and father built. He finds his mother's locket which contains pictures of the Claytons, though Tarzan does not recognize them. He also finds his father's dagger and finds that it can be used as a weapon. He fashions a belt for himself and carries the dagger with him. While foraging for food, Kala is attacked and shot with arrows by a native hunting tribe. Tarzan rushes to her rescue and tries to fight off the attackers. One of the hunters, wielding a spear, lunges at Tarzan who dodges the point as it hits his already dying mother. The blow finishes Kala and Tarzan, in a rage, breaks the man's back over his shoulder. As he crouches over Kala's body, Tarzan begins to moan and scream, a mournful sound heard by the rest of the hunters.
Many years later another hunting party, this time English, enters the jungle near Tarzan's home. The party is led by the vicious Major Jack Downing (Nigel Davenport), a game hunter who's been charged with bringing primates back to Britain, alive or dead, to be studied or stuffed for museum displays. Among the party are Capitaine Phillippe d'Arnot (Ian Holm) and Sir Evelyn Bount (John Wells), both contemptuous of Downing's sadism. They find the Clayton home and set up camp but are later attacked by a native tribe. Downing is killed immediately along with most of the guides. D'Arnot is wounded when an arrow pierces his lower abdomen. He manages to escape the fray but is followed by a few natives. He loses them as he hides in a large tree and, shortly after pulling the arrow from his body, is found by an adult Tarzan (Christopher Lambert) and a few apes.
Tarzan brings d'Arnot to the ape camp and tends to his wounds, much to White Eyes' disapproval. Tarzan also fends off the curiosity of his fellow apes while d'Arnot recovers. During this time he discovers that Tarzan is a talented mimic who can imitate most of the animals in the jungle along with d'Arnot's mumblings. One day, Tarzan begins to hum the same tune d'Arnot does. When d'Arnot finds that Tarzan has the ability to speak, he is delighted and teaches Tarzan rudimentary English. Tarzan shows d'Arnot the locket and takes him to the Clayton home where he found it. There, d'Arnot discovers Tarzan's ancestry from John Sr.'s journal and resolves to bring Tarzan (whom d'Arnot now calls "Jean"; the French pronunciation of "John") home to his family in England.
However, Tarzan's first priority is to gain leadership of the ape tribe. Having been persecuted by White Eyes his entire life, Tarzan challenges the older ape to a showdown. The two engage in a vicious fight that continues through the jungle while d'Arnot and the other apes look on. Tarzan and White Eyes fall into a large pool and disappear under water. White Eyes appears at the surface revealing a large stab wound in his body. Tarzan emerges from the water victorious and is respectfully greeted by the other apes as their leader.
D'Arnot continues to educate Tarzan, though he is met with resistance at first. Tarzan finds it difficult to easily discard his ape upbringing. Eventually, d'Arnot decides that John is civilized enough to return to the world & takes him out to a small settlement on the edge of the jungle where they make their way, by boat, back to England.
Upon their return, John is brought to the home of his grandfather, the Sixth Earl of Greystoke, who is deteriorating a bit from age and exhibits some eccentric behavior, presumably from the loss of his son and daughter-in-law. Lord Greystoke welcomes his grandson, though sometimes forgetting that he is not his son returned. John meets Lord Greystoke's young ward, Jane Porter (Andie MacDowell), who is visiting from America and is engaged to another man. Finding it difficult to be seen as anything but a novelty in a social sense, and with his behavior seen as threatening and savage, John falls into a depression. His mood is lifted slightly when he befriends a mentally disabled worker on the Greystoke estate and, while in his company, is able to relax and act naturally.
Jane takes it upon herself to teach John more English, French, and social skills such as table manners and dancing. The two become very close and develop feelings for one another, making love one evening in secret. Eventually, Jane leaves her fiance with the intent to marry John.
During a large ball honoring John's return to the family, Lord Greystoke sneaks away from the crowd, appearing to enjoy a renewed vigor at the return of his grandson. Reminiscing of a childhood game he used to play, he uses a silver tray as a toboggan and slides down the grand staircase, only to crash into the railing at the bottom. John discovers him and holds his grandfather as he dies in his arms, apparently of a head injury. John reacts similarly as he did to Kala's death, devastated and confused by the loss and frustrated with his displacement in society.
John is later on hand to cut the ribbon for a new wing at the Natural History Museum -- his grandfather was the chief benefactor. Bothered by the exhibits of preserved animals, John sneaks away into an alley and comes across an institute for the study of primates next to the museum. When he enters the lab he discovers, to his horror, several small monkeys and a gorilla that have been dissected. Several other primates are kept alive in cages, the largest of which contains an ape who John immediately recognizes as his adoptive father, Silverbeard. John frees him and the other primates and escapes. At the museum, Jane, d'Arnot, and Blount are informed by the police that John is running through the streets with an ape. John runs to a park where he and Silverbeard climb a tree as they are surrounded by police and a crowd. When Jane, d'Arnot, and Blount arrive, Blount gives the word to a rifleman who fatally shoots Silverbeard. John becomes enraged and sad, yelling that the ape was his father from Africa.
That night, everyone discusses what is to be done with the distraught John. Jane and d'Arnot agree that he should be sent back to Africa. Because of the dual nature in his mind, he cannot adapt to life in the civilized world. Blount disagrees, stating that John is the last surviving member of the Greystoke family and that the legacy must be secured for the future. Suddenly, they hear John screaming and see him driving a team of horses around the circle outside. When he comes in, covered in mud, Blount assertively tells him that he must resume his life as a Scot Lord. John growls and forces Blount into a chair. He presses a hand to Blount's chest and tells him, "One half of me is Greystoke. The other half is wild!"
John is taken back to Africa. At the waterfall in his former territory, he quickly meets another gorilla and runs off into the rainforest, while Jane and d'Arnot watch him leave.