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 and Namor to save their teammates.

The Sub-Mariner returns, but this time he needs help to defeat Attuma, who has taken over Atlantis. While Reed, Susan, and Ben head off to help free Atlantis, Johnny stays behind and faces an attack on the city from a giant mutant whale.