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.

Ships are being damaged by an unknown agent hidden at a depth impossible for the Navy to reach, where the Cetacean finds an automated robot, the Mud Worm. It is programmed to crawl along the sea bottom collecting mineral samples, but was damaged, and now resists all efforts to approach it with deadly force. Mark manages to shut down the robot and haul it inside the Cetacean, but when it reactivates, it will destroy everything in its path in an effort to escape.