Tarzan (Johnny Weissmuller), that denizen of the African jungles, lives near a river which stretches for many miles through the continent until it reaches the sea. It is there the tribe of the Aquatanians live in an apparent paradise, diving for pearls and fishing for their livelihood, enjoying the sunny climes and falling in love. Yet it used to be a lot better, as now they have to worship their god who stands at the top of the islands and makes demands on the natives to provide pearls for him, a duty the locals feel they cannot turn down. But when one of the young women, Mara (Linda Christian) has to be married to it...
Let's just say she's not keen and makes good her escape, diving off a high rock and swimming her way to freedom up that river where she arrives chez Tarzan and the adventure begins. This one would be a little more poignant than the others, not because of the plot but because it was to be the last time Weissmuller appeared in the role that had made him an international star; sure, he had made his name thanks to his record-breaking skills as a swimmer, but if he's recalled today it'll be as Tarzan. After this, life didn't work out entirely well for him, though he did continue in movies with the Jungle Jim B-movie franchise.
For a while at any rate, until other business pursuits took up his time. So as an apparent tribute to the star, Tarzan and the Mermaids featured him undertaking more swimming scenes than he had in the rest of the series put together, seemingly spending half the movie freestyling through the waves, diving off cliffs and venturing to the sea bed where he could get up to such business as battling a giant octopus for no other reason than the plot needed a spot of peril during the finale. The Ape Man was convinced to go to Aquatania when Mara spilled the beans about the repressive regime there, so he and Jane (Brenda Joyce, who would continue with Weissmuller's replacement Lex Barker) set off.
Feeling the need to right wrongs, and particularly rescue Mara from the heavies who kidnap her to return to the island she fled from. That island was a curious one considering it was meant to be African, resembling Hawaii instead though actually filmed in Mexico, yet another example of the geography of the forties Tarzan efforts growing stranger the further they went along. As for the godlike presence there, it is of course nothing of the sort and there's an avaricious pearl trader, Varga (Fernando Wagner), under the costume, in cahoots with the high priest Palanth, played by a familiar face in these things George Zucco, himself no stranger to outlandish costumes.
All of which he wore with his accustomed air of imperious regality, which was why he was so much fun to watch in these works. What wasn't fun was the story that went about concerning the high cliff dive Tarzan achieves near the finale; he had to prove himself as the best at that daredevil activity so took the highest spot to leap from and obviously the studio wouldn't have allowed Weissmuller to carry out the stunt even if he'd wanted to, so his stunt double did it. To all appearances this man genuinely did succeed without trick photography, but then an urban myth began to circulate about the diver actually being killed in the process of the jump, not something that seems to have any basis in truth, but still hangs around the yarns told about this run of movies featuring the Edgar Rice Burroughs character. Whatever, Weissmuller did come across as enjoying himself, maybe thanks to all that swimming, and if you could put up with singer John Laurenz as a Boy substitute (many cannot) then the skill of veteran director Robert Florey kept it rattling along. Music by Dimitri Tiomkin.