Sheffield, seven years old at the time of filming, won the role - with Weissmuller's approval - that over 300 youngsters had tested for.
While a handsome-looking production, the costs of Tarzan Finds a Son! were kept at a tidy $880,000, thanks to the extensive use of stock footage from earlier series entries as well as from Trader Horn (1931). His machinations lead to a betrayal of Tarzan by Jane, and the departing safari's capture by a savage tribe, setting up a rousing rescue attempt courtesy of Tarzan & Son.
Thomas wears down Jane's effort to conceal Boy's identity Austin, with control of the estate as his motive, wheedles her into believing that a return to civilization would be in his best interests. Along for the trip is the conniving Austin Lancing (Ian Hunter) and his wife (Frieda Inescort), who stand to inherit the Greystoke fortune. The idyll is eventually interrupted by a safari mounted by Lancing's uncle Thomas (Henry Stephenson), desperate for some clue as to the survival of his nephew's family. In the five years that pass, the child develops his adoptive father's facility at swinging through the trees and giving out with the familiar jungle call, as well as a penchant for getting himself into danger. After the ape-man retrieves the foundling, the immediately smitten Jane urges his adoption. (This would be the only time that the MGM Tarzan films would make a reference to Tarzan's aristocratic roots, a detail from Burroughs' novels.) The aircraft crashes in the proximity of Tarzan and Jane's jungle abode, leaving Lancing's infant son as the sole survivor. As a result, the screenplay of Tarzan Finds a Son! opens with a fateful plane ride to Cape Town for the little family of Richard Lancing (Morton Lowry), the heir apparent to the title and fortunes of the earl of Greystoke.
While Burroughs' tales had no moral viewpoint on Tarzan and Jane having a son of their own, Hollywood's Production Code was more leery of their somewhat nontraditional family arrangement. As evidenced by the title, the fourth Weissmuller/O'Sullivan opus, Tarzan Finds a Son! (1939), has the distinction of introducing Johnny Sheffield to the series' mythos as "Boy," beginning the young actor's eight-year association with the role. Although filmmakers have tried their hand at adapting Burroughs' fiction since the days of the silents, no such efforts before or since captured the public's fancy like those pairing Johnny Weissmuller as the "Lord of the Apes" with Maureen O'Sullivan as Jane Parker, the Englishwoman who tamed his heart. One of MGM's most crowd-pleasing franchises developed during the 1930s was its series of jungle adventures based on Edgar Rice Burroughs' Tarzan. Tarzan then forgives Jane, and the reunited family returns home.
Boy survives the perils of the jungle to free Tarzan, and they return with an army of elephants to trample the cannibal village. Soon after, the expedition is captured by a band of savage cannibals and Jane risks her own life so that Boy can escape.
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. Going against Tarzan's wishes, Jane traps him in a grotto and brings the boy to the Lancings. When Thomas objects, Austin orders him held prisoner and convinces Jane to give the boy up. 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. 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. 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 mischevious boy when a safari led by Sir Thomas Lancing arrives in search of his missing relatives. 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.