A water-breathing human, the sole survivor of an underwater civilization called Atlantis, emerges from the ocean and has to learn to deal with the air-breathers on the surface.

The Cetacean is called upon to recover a space probe that crashes in the sea, and Mark soon learns that the probe is covered with thousands of tiny alien beings. They intend to learn all they can about the human race by invading host bodies and controlling them. Mark must return them to space before they destroy all humanity.