Mark investigates the disappearance of three scuba divers, two of whom are 'replaced' by waterbourne aliens (Tiffany Bolling as Lioa / Dilly and Burr DeBenning as Xos / Chazz). The aliens assignment is to scout Earth and check its defense capabilities. They also give the impression that Mark may also be one of them, specially bred for Earth infiltration, but this is not specifically confirmed.

When a space capsule crashes in the ocean near the Cetacean, Mark and Elizabeth agree to investigate. When they arrive, Mark is rendered unconscious by a screeching sound. They discover hundreds of strange, blue, intelligent spores that have taken over Mark's body, attempting to find a way back into space. However, they do not know that Mark will die if he does not return to water within a few hours.

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.