The TARDIS materializes on Coralee, a water world colonised by humans and intelligent dolphins. Apart from minor social troubles caused by a Dreekan drug and voodoo cult, it appears to be the perfect place for the Doctor and Ace to relax and recuperate from their traumatic experiences in Victorian London. The Doctor, however, finds children playing with an odd shell on the beach, and later that night in the hotel bar, he and Ace meet a drunken travel writer named Edwin Bryce who tells them that Coralee has a history of war and genocide. The native species died out centuries ago, leaving only underwater ruins and legends of a monstrous species called the Krill which devastated whole worlds. The Doctor says nothing to Ace for fear of ruining her holiday, but he suspects that something terrible will soon happen in the colony. The next day, while Ace is off enjoying herself, the Doctor breaks into the main administration building, hacks into the computers, forges documents identifying himself as an investigator for InterOceanic, and access all data stored on the Krill and on any recent odd events in the colony.
While preparing to leave, the Doctor finds the colony co-ordinator, Brenda Mulholland, lying unconscious in a meeting room, where she was attacked by an odd-smelling creature which has been glimpsed lurking in the service tunnels of the colony. The colony's chief engineer, Philip Garret, dismisses as ridiculous the claim that there could be an alien roaming the colony undetected. Mulholland accepts the Doctor's credentials and agrees to let him investigate the wreck of the Hyperion Dawn, a deep-sea cable-laying vessel which was recently found abandoned and torn apart. Meanwhile, the alien in the colony sends word of the Doctor's arrival to a ship in the asteroid belt crewed by Cythosi, a race of carnivorous warriors who tend to punish disobedient slaves by eating them. The Cythosi have been led to Coralee by a traitorous cetacean, Blu'ip, who has not forgiven humanity for their centuries of oppression and murder of his kind; following their agent's report, they prepare to invade the colony ahead of schedule if necessary. Their human slaves, led by the rabble-rousing Peck, are plotting rebellion, and General Mottrack, sensing the growing unrest, sets the ship's service drones on permanent defensive status; anyone detected in an unauthorised area, even another Cythosi, will be killed immediately.
Ace finds Edwin Bryce preparing to take a submersible tour of the underwater cities to gather material for his book. Although unimpressed by Bryce, she finds the ship's captain Rajiid attractive and joins the tour. As the cetacean R'tk'tk guides them through the abadoned cities, they find an escape pod containing Holly Relf, the sole survivor of the attack on the Hyperion Dawn. She is in a state of shock, and as they try to revive her, the submersible is attacked by vicious predators like humanoid fish, which are strong enough to claw their way through the submersible's steel hull. Rajiid releases blood from the sub's storage tanks to attract a giant Ramora eel; ordinary a tourist attraction, it now provides a diversion which enables Rajiid to escape back to shore while the monsters attack the eel. Upon returning to the colony, however, Rajiid and R'tk'tk are taken in for questioning by Garrett, who is almost single-handedly responsible for founding the colony and thus holds unprecedented authority over police operations.
The Doctor joins Professor MacKenzie and his aide Q'ilp, who are heading out for an archaeological dig and will be passing the wreck of the Hyperion Dawn on the way. The Doctor determines that the talon which he found on the beach earlier belongs to the same creatures which tore apart the Dawn. Mackenzie, eager to secure funding from InterOceanic, proudly shows the Doctor around the dig, which he believes to be a temple. The hieroglyphs describe the legends of the Krill, but MacKenzie dismisses them as superstition; as far as he's concerned, Bryce is a morbid sensationalist and the civilisation on Coralee died out naturally. The Doctor finds a dead alien body in the underwater tunnels, confiscates the encrypted data pad it was clutching and has the body taken back to the colony for an autopsy. He later has Q'ilp take him back to the dig for another look around, and finds that the dead alien was killed by a booby trap while trying to break into a hidden room. Inside, the Doctor finds the temple's inner sanctum and a symbol of worship for the planet's warlike civilisation -- an ancient biological weapon of some kind.
Bryce, seeing strangely subdued by his experience, is also taken in for questioning about the attack on the submersible. Ace returns to the admin block to find out why her friends are being held, but she is attacked by the same creature which assaulted Mulholland earlier. She manages to escape but is caught by Garrett, who accuses her of sabotage and inventing a story about an alien to divert suspicion. Mulholland dismisses his paranoid claims and releases Ace and her friends, and Ace and Rajiid go to the beach to discuss the situation; there, however, they find that the body of one of their attackers has washed up on shore, and take it back to Rajiid's workshop to study it. Once there it revives and attempts to kill them, but the Doctor arrives just in time with the colony's police forces. Repeated, concentrated fire from the police guns eventually kills the creature, but the police then confiscate the weapon which the Doctor found at the dig. They claim that unauthorised weapons are not permitted on the colony, but the Doctor suspects that they were ordered to confiscate it by Garrett.
The Doctor discovers that the monster's body consists of undifferentiated tissue, which makes it nearly impossible to kill; even this close to death, it is secreting larval tissue in an attempt to reconstruct a pupa around itself and heal. The Doctor rips out its internal tissue, finally killing it, and confronts the nervous Bryce, who admits that he's terrified that the Krill may still be alive. All of the legends he's collected identify them as creatures of fierce, unstoppable rage, which fed on radiation and could survive even exposure to vacuum and atmospheric re-entry. Bryce, as requested by Garrett, attempts to tell him that the Doctor is investigating the Krill, but upon entering Garrett's office he finds the half-eaten body of the alien from the dig, and is killed by the alien which attacked Mulholland and Ace. Meanwhile, a police sub sent out by Garrett to investigate the attacks is diverted onto a new course, and by the time the crew realize that the navigational signal was unauthorised the ship's reactors have been set to overload...
The Doctor finally accesses the alien's data pad and discovers that it contains a map indicating the locations of Krill egg nests across the planet -- and that the colony's reactor has been built directly atop one. The police sub explodes near another concentration of Krill eggs, awakening the Krill within, but thanks to the Doctor's warning Mulholland is able to raise the colony's storm shutters in time. Millions of Krill rise from the waves, slaughter the crew and passengers in the cruise liners trapped outside, and begin to rip apart the storm shutters, trying to claw their way into the colony. The Doctor believes that the weapon from the dig may be the only thing that can stop them, but Garrett has it -- and when the Doctor tries to confront him, he finds only the alien which killed Bryce. As the alien flees, the Doctor realizes that it's of the same species as the dead alien from the dig -- and that, unlikely as it may seem, it's Garrett.
Garrett, suffering from a severe identity crisis and unsure whether he's human or alien, stumbles across a group of Dreekan cultists who believe him to be the embodiment of their god Treeka'dwra, the beast within. The Krill break into the colony through the dolphin tunnels, and although Ace and Rajiid manage to fight off some of them, they appear about to overrun the colony -- when Garrett suddenly reappears with his new followers and triggers the biogenic weapon. The humans and cetaceans in the colony are knocked out, and recover to find that the Krill have all been killed. Garrett and his followers have fled to a Dreekan temple in the forests outside the colony, but there's no time to search for him; a hurricane is approaching, and due to the damage inflicted on the storm shutters by the Krill, the colony will be unable to withstand the storm.
A fleet of Cythosi shuttles arrives from the warship in the asteroid belt, and Mottrack contacts Mulholland, claiming to offer assistance. The Doctor, however, suspects differently, especially when he realizes that the dead alien from the dig was a Cythosi. The Cythosi help to restore the colony's storm shuttles and force field, but they are doing so in order to distract attention from their real activities. Garrett is in fact a deep-cover Cythosi agent named Skuarte, who has been living as a human for twenty years; he helped to found the colony on Coralee, under orders from Cythosi high command, and thanks to the work of the human colonists all the Cythosi need do now is land on the planet and harvest the Krill eggs. Mottrack's adjutant, Bisoncawl, leads the Doctor to the colony's reactor, where the Cythosi are using a power tap to the reactor to provide energy for a mass transportation beam. The Krill are to be used as the ultimate weapon; after they have destroyed all life on a target planet, the Cythosi will use the weapon from the dig to wipe out the Krill, sterilising the planet. But now Garrett, and the weapon, have vanished...
MacKenzie leads Ace and Rajiid to the Dreekan temple, but they are forced to shelter inside when the hurricane hits, and are captured by the cultists. The pressure of twenty years under deep cover has driven Garrett insane; he is now unsure whether he is Garrett, Skuarte, or the Treeka'dwra. He kills MacKenzie as a sacrifice to the Dreekan voodoo gods, releases Ace and Rajiid so they can help him, and then rushes ahead to the colony, leaving them trapped in the forest as the hurricane rages across the island. Upon arriving at the colony, Garrett sabotages the power relay unit and then transports himself to the Cythosi mothership. As the reactor begins to overload, the Doctor, realizing that its explosion will reactivate the Krill eggs and result in the destruction of the colony, sends the Cythosi to safety while he tries to transport the rest of the eggs into the mothership's holding tank. He succeeds, and transports himself to safety with them just as the reactor explodes. Unfortunately, he has miscalculated slightly, and trace radiation causes the eggs to begin hatching within the holding tanks.
Before the Doctor can convince Mottrack of the danger, the human slaves take advantage of the distraction to begin their rebellion, and seize control of the lower decks. Garrett, convinced that he is the Treeka'dwra, holy saviour and bringer of the fires of retribution, blasts open the holding tanks and releases the Krill upon the Cythosi ship. The Cythosi seal their ship's bulkheads, trapping their rebellious slaves with the Krill, and the Doctor is forced to lead the surviving slaves through the service shafts. They are attacked by service robots programmed to kill, but Peck dies holding the robots off while the others get to the shuttle bay. The Krill break through the bulkheads and attack the Cythosi, but avoid Garrett while he carries the biogenic weapon, thus reinforcing his deluded belief that he is the Treeka'dwra. Mottrack blames Garrett for the destruction of his ship and attacks him, and in the confusion, Blu'ip destroys the weapon and all but one of the ship's shuttles, intending to trap them all aboard. When the ship goes out of control and crashes into the colony, all of the humans will be killed, either by the explosion or by the Krill awakened by the radiation. Blu'ip then attempts to escape in the remaining shuttle, only to be attacked and killed by a Krill.
The Doctor sends the slaves to safety in the shuttle while he remains aboard the mothership, intending to crash it into the asteroid belt. Bisoncawl remains to protect him and gives his own life to hold off the Krill until the Doctor has completed his mission. The Doctor then finds a dying Krill, carries it to the cargo bay launch pad, and buries himself in its larval secretions after triggering the launch protocol. Protected by the impervious pupal shell, he is ejected into space as the Cythosi ship crashes into the asteroid belt and explodes. When the pupa lands on Coralee the Doctor is found and removed, and reunited with Ace, who managed to get back to the colony with Rajiid during the eye of the storm. The Doctor and Ace remain for a while to ensure that the colony is recovering, and then depart -- unaware that the Krill have survived the destruction of the Cythosi ship and are now in pupal form, attached to the asteroids, awaiting only a surge of radiation to bring them back to life...
Source: Cameron Dixon