Original Airdate: ?/?/1983
An informant tells Man-At-Arms and King Randor about six ships that have recently disappeared. Later, Adam and Man-At-Arms look at a screen on a naval ship. Man-At-Arms wants to use the remote-controlled ship to investigate all the recent disappearances. When Adam, Man-At-Arms and Cringer set off in the ship, they spot an underwater city on the monitor.
A mechanical spinning device emerges from the city and begins creating a whirlpool. The whirlpool pulls them underwater, where they find themselves inside the ancient kingdom of Aquatica. Mer-Man appears holding the Pearl of Power and uses its energy to bind the heroes. Mer-Man tells the Aquaticans that the heroes took Princess Nami. In an arena the heroes do battle with a giant crab creature, and after defeating it they are saved by Shalandor.
The heroes find Princess Nami trapped in a glass cage, and do battle with a giant guardian beast. Freeing Nami they head back to Aquatica and de-throne Mer-Man, who turns the whirlpool device on the city itself, but He-Man manages to save the city.
Synopsis by Zadoc Angell