Mutant humans who live underwater attack the people of N'Yark. Among other hostile acts they dam the Verrazano Narrows so as to flood the city, allowing them to take it over as a new homeland.
When an Ultra City professor is trapped under water, Tom enlists a dolphin to help rescue him. Tom exhausts himself sealing a lava flow that the professor found, but the dolphin returns him safe to shore.
The tribesmen fight back against the fishmen but Samson is hampered in fighting Nephtoon beacause he repeatedly needs to surface for air. Mindor outfits Samson with an ancient SCUBA outfit, after which Samson brings Nephtoon to the surface, where he forces him to call off the attack and dismantle the dam.