Elizabeth is kidnapped, one of dozens of scientists held prisoner on the island of Felicitos, controlled by special mineral springs that render their victims completely happy and compliant. Dr. Smith is using them to build a rocket to take her away from our troubled planet in search of some better world, and Mark must find a way to counter the brain-washing and free the captives.
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.
A human scientist capable of living underwater discovers similar alien life inhabiting human bodies, and looks to them for a clue to his origin.
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.
