When the plane carrying Richard Lancing and his family crashes over the African jungle, the lone survivor is Lancing's infant son, whom jungle inhabitants Tarzan and Jane find and rear as their own.

Five years after they find the child, whom they call "Boy," Jane is just beginning to realize that the jungle is a dangerous place in which to rear a mischievous boy when a safari led by Sir Thomas Lancing arrives in search of his missing relatives.

After finding the plane wreckage, Sir Thomas insists upon continuing the search, although his greedy cousin Austin, who stands to inherit half the vast Lancing estate once Richard is declared dead, is content to accept Jane's explanation that Richard, his wife and child perished in the crash.

Sir Thomas is unconvinced, however, and when he notices the resemblance between Boy and the Lancing family, Austin and his snooty wife propose that they take the boy as their ward, thus ensuring their control over the Lancing estate.

When Thomas objects, Austin orders him held prisoner and convinces Jane to give the boy up.

Going against Tarzan's wishes, Jane traps him in a grotto and brings the boy to the Lancings.  As Jane leads the expedition through the jungle, Thomas warns Jane of Boy's danger, but Austin kills him before he can go for help.

Soon after, the expedition is captured by a band of savage cannibals and Jane risks her own life so that Boy can escape.  Boy survives the perils of the jungle to free Tarzan, and they return with an army of elephants to trample the cannibal village.  Tarzan then forgives Jane, and the reunited family returns home.

Notes
The working title of this film was Tarzan in Exile.  It was also reviewed as Tarzan Finds A Son!  According to news items in HR, the underwater scenes were shot at Crystal Springs and Silver Springs, Florida, and additional location shots were filmed at Ocala, Florida.  Other items in HR note that the elephants featured in the stampede scene were provided by the Hagenbeck-Wallace Circus and the film's shooting schedule was modified to allow star Maureen O'Sullivan, who was pregnant at the time, to finish shooting before her condition prevented her from doing so.  The studio originally wanted to replace O'Sullivan because of her pregnancy, but exhibitors insisted that she reprise her role as "Jane."

Also, according to studio records, in the original screenplay for the film, "Jane" is hit in the back by a spear while rescuing "Boy," and she dies soon after Tarzan finds her.  In a letter to an MGM official dated January 6, 1939, Edgar Rice Burroughs stated that he regretted the killing of Jane in the screenplay and feared that the ending would have a bad result at the box-office.  An "alternate ending," which corresponds to the ending used in the final film, is included in a screenplay dated January 31, 1939.  Modern sources state that the first ending was shot and previewed, and that audiences disapproved of "Jane's" death.