Namor the Sub-Mariner arrives with a decree banning humankind from the oceans. Susan and Johnny go underwater to negotiate, but are taken prisoner in Atlantis. Ben and Reed must fight their way through sea monsters, Atlantean soldiers, and Namor to save their teammates.

The Impossible Man is brought to Earth by one of Reed's space probes and immediately starts causing trouble for everyone. In the end, the Fantastic Four convince Impossible Man that Earth isn't as fun as he is.