Calabraxian terrorists threaten a colony of Sea-Devils on the planet Chosan, and the Earth Reptile Council turns to Irving Braxiatel for help, as they fear that the terrorists have an inside contact. Braxiatel sends Benny in undercover, since she’s visited Chosan before and can claim to be visiting her friend, Clinician Nedda. Benny reluctantly accepts the mission, though this means being separated from Peter and from Jason, who’s just returned to the Braxiatel Collection after undergoing treatment for a series of crippling migraines. Benny hires Carver, the young manager of a transport company, to take her to the Sea-Devil colony; in fact, Carver is the leader of the terrorist cell, and Benny wants to keep a close eye on her. However, Carver has her own plans, and when the Sea-Devils lower their security grid to let Benny in, Carver slips into the colony behind her. Once inside, Carver kills a security guard who is unfortunate enough to stumble across her, and uses the authorisation codes provided by her contact to seal the dock, ensuring that nobody can leave the colony.

The terrorists are not the only threat facing the colony; the marine life on the planet is hostile, and the colonists have been falling ill. Nedda and her assistant, Ressix, have discovered an alien protein in the planet’s oceans, and believe that it is causing mutations in the Sea-Devils’ messenger RNA. Ressix believes that the mutations could progress to a point at which the body adapts to the protein’s presence; however, this is only a theory, as exposure to the protein has invariably proven fatal. The colony’s leader, Lurnix, dismisses Nedda’s concerns as irrelevant and advises her to put her faith in their people’s gods, as he does. Nedda is concerned, but is unaware that Lurnix is in fact the insider who is helping Carver to destroy the colony. Elsewhere, Carver plants a bomb and sets it on a timer, giving herself enough time to get clear, but as she prepares to return to her waiting submarine, it is attacked by sharks and destroyed with all hands on board. Carver starts looking for the cave systems said to connect the colony to the Calabraxian shoreline.

While searching for Nedda’s laboratories, Benny encounters a Sea-Devil wandering the corridors, rambling about “the wrong water.” Eventually, she finds her old friend, who offers to introduce her to Lurnix. On the way to the colony’s command centre, however, they find the body of the security guard, and when Nedda tries to sound the alarm, she finds that she’s been locked out of the computer systems. Benny admits that she may know what’s going on, but insists that she must speak with Lurnix before she can explain; however, when she finally does meet Lurnix at the command centre, she’s disturbed to recognise him as the rambling figure she encountered earlier. Benny tells Lurnix and Nedda the truth about her mission, and, in as non-accusatory a manner as possible, asks Lurnix how things are going in the colony; however, Lurnix dismisses her, refusing to discuss Earth Reptile matters with an outsider.

Benny tells Nedda about Lurnix’s earlier behaviour and admits that the Earth Reptile Council suspects him of aiding the terrorists. In turn, Nedda and Ressix tell Benny about the alien protein they’ve discovered. Ressix continues to work on analysing the protein while Benny and Nedda set off to confront Lurnix, but when they run into him in the shrine to the Sea-Devils’ gods, he is rambling and disorientated. Nedda remains to keep an eye on Lurnix while Benny fetches help, but outside the shrine, Benny is confronted by Carver, who overheard their conversation and now knows that Benny came here to spy on her. Furious, she forces Benny into the caves at gunpoint, intending to tie her up with the bomb.

Meanwhile, Ressix makes a disturbing discovery: the alien protein is common to all sea life on Chosan, and may be a vital part of life on this planet. He sets off to inform Nedda, and arrives in the shrine just as Lurnix attempts to throttle her to death. Lurnix claims to be the alien protein, and when Ressix tries to shoot him, he finds that his gun has no effect. Nevertheless, Ressix remains to hold Lurnix back, giving his own life to let Nedda escape. After killing Ressix, Lurnix returns to the command centre and orders the staff to deactivate the colony’s security grid, killing the chief technician when he hesitates. Lurnix then sets off to look for Nedda, who arrives at the command centre moments after he’s gone, looking for Benny. She easily convinces the technicians that Lurnix is not himself, and they turn the security grid back on. They are unable to locate Benny, but do detect a partial security blackout which Nedda decides to investigate.

Carver ties Benny to the bomb with which she intends to destroy the colony. Carver’s family was killed in the war between Calabraxia and Pevena, and she joined the Calabraxian military while still a teenager -- but when the Pevenan plan to destroy all of Calabraxia was revealed, both sides were so horrified by how far things had gone that they agreed to a negotiated peace. Unable to vent her anger upon her traditional enemy, Carver has turned it upon the Sea-Devils, blaming them for taking away hard-won Calabraxian territory. When the security grid goes down, Carver gags Benny, activates the bomb and sets off to look for a way out. However, Lurnix finds her first. The hostility of the human colonists and the local sea life had convinced Lurnix that his people were not meant to live on this planet, and he had been working with Carver to destroy the colony base and force his people to move on to another world; however, the thing he has become now has another agenda, and it attacks Carver, telling her that this planet doesn’t belong to her either.

Nedda finds and releases Benny, and calls in a security team to defuse the bomb. She and Benny then return to Nedda’s lab, where they find the results of Ressix’s research and conclude that the alien protein is alive and sentient -- and that Lurnix has been subsumed into its gestalt. Benny and Nedda confront Lurnix in the command centre, where they find that he has dismissed the base’s technicians and ordered the computers to open the aqua-locks. The colony will be flooded within half an hour, and every Sea-Devil will be exposed to the alien protein. The first Sea-Devils who encountered the protein died due to its influence, but it has now learned how to adapt to the Sea-Devils’ bodies -- and once it possesses them, it will use them as its army to wipe out the humans who have tried to colonise its world.

Benny realises that, although some of Lurnix’s memories remain, the alien can no longer recall the details of his deal with Carver. While Lurnix waits complacently for the base to flood, Benny sends Nedda to evacuate the Sea-Devils while she tracks down Carver. Carver is wounded but alive, and, though tempted to leave her here to die, Benny helps her to reach the caves in return for the bomb’s remote activator. Once the Sea-Devils have all evacuated, Benny detonates the bomb prematurely, destroying the base and killing Lurnix with it, thus depriving the alien of the one Sea-Devil it had under its control.