With Jane and Tarzan (Johnny Weissmuller) away, their adopted son Boy (Johnny Sheffield) is exploring the jungle around their home, and stops by the cliff overlooking a lost city. However, he gets too bold and begins to climb down to it, then stumbles and is left in a precarious position that is only solved when one of the city's residents, Zandra (Frances Gifford), helps him, though now they are both stuck. Luckily Tarzan was in the area and he recruits Boy's elephant friend to pull them both out of harm's way. Zandra is most grateful, but Tarzan would rather leave her well alone...
This was the first Tarzan movie to be made by RKO after a run with MGM, and only Weissmuller and Sheffield saw their services retained as Maureen O'Sullivan had lost interest after their last outing together, setting her sights on fresh challenges. Not so for her co-stars, well into being typecast for however long their screen careers would last, and Jane was written out of the story this time around to be replaced by Zandra, essayed by Gifford who did have a link to the series in that she had played the Jungle Girl in a serial a few years before - an Edgar Rice Burroughs character, like Tarzan.
Whether it was intended to have Gifford be a recurring character to replace Jane seems unlikely, and soon after another actress was brought in to fill O'Sullivan's shoes. Here, on the other hand, other things were on the filmmakers' minds as the famed jungle hero was signed up to help the war effort as many series characters were wont to do, but Tarzan takes some persuading. This was a clunking way of introducing the concept that American isolationism was A Bad Thing, and they should put aside their reservations to join up with the fight against the Axis powers, but the theme was relevant at the time.
And after all, it did lead up to one of the most celebrated lines in the franchise's history, although you had to wait for it as the Ape Man proves extremely reluctant to help the lost city rid themselves of the Nazis who have arrived there seeking their precious oil reserves. At first he even helps a Nazi who has parachuted into their part of the jungle back to health, as if it was his duty to look after whoever he found in need of his assistance, but the soldier (Rex Williams) is more interested in getting his radio message back to his superiors, which is tricky when Cheeta keeps stealing an important component.
Boy is keen for his father to change his mind, so encourages Zandra to basically act as a surrogate Jane until he relents, but it takes the soldier's violent actions to do so. When Boy is kidnapped, it's the final straw, and here comes that line: "Now... Tarzan make war!", which made audiences of the day cheer and is still pretty stirring stuff now. It is a little disturbing seeing the noble savage start actually killing people, even if they are Nazis, as it seems out of character - and both Boy and (absurdly) Cheeta take up firearms to gun down the bad guys too. Of course, Tarzan Triumphs gained a little more cult cachet when the scenes where the three leads are tied up were extracted for the Christina Lindberg sexploitation flick Exposed, fuelling her heroine's bondage fantasies, but if there's anything unwholesome here it's more that slaughter at the end. Cheeta getting mistaken for Adolf Hitler was a nice, corny touch though. Music by Paul Sawtell.