Susan and Johnny witness the arrival of Namor, the Sub-Mariner, who bans humankind from using the oceans of the world. Susan takes Johnny (who hates the water with a passion) under the sea, hoping to reason with Namor. Instead, the siblings become prisoners of Namor in the fabled city of Atlantis. Reed and Ben fight their way through a swath of undersea monsters to come to their rescue, but Namor proves to be a formidable foe. And even worse, impressed by Susan's eloquent pleas for peace, he decides to keep her in Atlantis as an ambassador.