The novel tells the story of John Clayton III. John and Alice (Rutherford) Clayton II, Lord and Lady Greystoke of England, are marooned in the western coastal jungles of equatorial Africa in 1888. In September 1889 their son John Clayton III is born. At one year old his mother dies, and soon thereafter his father is killed by the savage king ape Kerchak. The infant is then adopted by the she-ape Kala.
Clayton is named "Tarzan" ("White Skin" in the ape language) and raised in ignorance of his human heritage.
As a boy, feeling alienated from his peers due to their physical differences, he discovers his true parents' cabin, where he first learns of others like himself in their books. Using basic primers with pictures, over many years he teaches himself to read English, but having never heard it, cannot speak it.
Upon his return from one visit to the cabin, he is attacked by a huge gorilla which he manages to kill with his father's knife, although he is terribly wounded in the struggle. As he grows up, Tarzan becomes a skilled hunter.
Later, a tribe of black Africans settle in the area, and Tarzan's adopted mother, Kala, is killed by one of its hunters. Avenging himself on the killer, Tarzan begins an antagonistic relationship with the tribe, raiding its village for weapons and practicing cruel pranks on them. They, in turn, regard him as an evil spirit and attempt to placate him. Some time after, Tarzan finds he has excited the jealousy of Kerchak, the ape leader, who finally attacks him. Tarzan kills Kerchak and takes his place as "king" of the apes.
At about the age of 20 a new party is marooned on the coast, including Jane Porter, the first white woman Tarzan has ever seen. Tarzan's cousin, William Cecil Clayton, unwitting usurper of the ape man's ancestral English estate, is also among the party. Tarzan spies on the newcomers, aids them in secret, and saves Jane from the perils of the jungle.
Among the party was French Naval Officer Paul D'Arnot. While rescuing D'Arnot from the natives, a rescue ship recovers the castaways. D'Arnot teaches Tarzan to speak French and offers to take Tarzan to the land of white men where he might connect with Jane again. On their journey, D'Arnot teaches him how to behave among white men. In the ensuing months, Tarzan eventually learns to speak English as well.
Ultimately, Tarzan travels to find Jane in Wisconsin, USA. Tarzan learns the bitter news that she has become engaged to William Clayton. Meanwhile, clues from his parents' cabin have enabled D'Arnot to prove Tarzan's true identity as John Clayton the Earl of Greystoke. Instead of reclaiming his inheritance from William, Tarzan chooses rather to conceal and renounce his heritage for the sake of Jane's happiness.