Professor Gamma, a mad scientist mutated into a fish-man by the radioactive undersea rubble of what had been his base until it was destroyed by the Fantastic Four, seeks vengeance with his new-found powers as the Gamma Ray.

The Mole Man is creating earthquakes and causing buildings to sink deep into the Earth. In addition, he and his Moloids kidnap Susan. The Mole Man as usual has been expecting the other three and sends them back to the surface to tell the Army not to get involved. They manage to halt them and seek an alternate entrance in the underworld. Johnny rescues Susan, then they penetrate the laboratory. They all return the buildings to the surface and escape the exploding caves.