The Doctor is tinkering with the TARDIS while Martha pesters him for lessons in how to drive it. Instead he offers to take her for breakfast at Tiffany’s. While she gets dressed up for a trip to New York he is setting course for the planet Arkon, and the Tiffany’s near the Robot Regent’s palace. Before the misunderstanding can be resolved the TARDIS lands on the planet Sunday. The Doctor steps out to see what kind of world he has landed on, finding it muddy and marsh-strewn. Losing his footing he reaches for the TARDIS only to send it slipping down a slope and into a small lake where it promptly sinks. Although the door was open the Doctor recognizes that the force field must have kicked in automatically as the lake is not being drained into his craft.

Martha steps out from her changing room and notices that the doors are open but the world outside is completely dark. Reaching her hand through the force field she has barely acknowledged that the darkness is actually water when her arm is grabbed and she is dragged into the lake.

The Doctor is perplexed as to how he will retrieve his craft but notices a spire of smoke. He decides to make for it when he encounters a teenage girl, Candice ‘Candy’ Kane. She has been searching for outsized ostrich eggs, a local delicacy, and leads him to the nearby settlement, Sunday City and explains the recent history of the settlers from Earth. Eight hundred people arrived in a spaceship, but soon after establishing the original Sunday City the planet was hit by a meteor. The ensuing tidal wave engulfed the settlement and drowned four hundred people including the entire leadership.

When the Doctor arrives at the new settlement he is taken to meet Ty Benson, a zoologist, and her colleague Col who are observing captured wildlife. These ‘otters’ are like Earth otters with bear’s faces. They are currently caged. The experiments show that the otters are usually aggressive and unintelligent when captured but grow more intelligent and placid for a few days, though their learning curve plateaus very quickly.

The Doctor’s arrival is immediately noticed by the leader of the settlement, a man called Pallister, who arrives with two men to arrest the Time Lord. However, the Doctor uses his psychic paper to persuade them that he is an adjudicator sent from Earth to investigate irregularities and he is given an office to work from. The Doctor uses his new position of authority to get half a dozen strong men and some ropes to rescue his craft, much to the perplexity of the settlers who imagine he arrived in a rocket.

Col and Candy are preparing to release some of the otters when the stocky teenager, Orlo, arrives with another otter in a sack.

Martha wakes up on a small wickerwork dome, apparently the natural habitation of the otters. As her eyes get used to the light she sees that she is lying among three human skeletons. The otters are busy around her when she notices a small pool in the middle of the chamber, from which a slimy creature is rising. It extends a tentacle that broadens to engulf her head.

Fortunately, the Doctor and his party are passing and burst into the chamber to rescue her. The Doctor uses the sonic screwdriver to release the slime creature’s grip. None of the other settlers has seen such a creature before. An unconscious Martha is taken to the settlement’s hospital and put under the care of Doctor Sam Hashmi. The Doctor is most worried about a series of puncture wounds on her temples. He leaves her to examine the captured otters and is curious to see that one has used its razor sharp claws to carve a rough diagram of the TARDIS.

Orlo, still out looking at otters, is amazed when they lead him to the water’s edge and show him that the water level is dropping and the space ship, One Small Step, on which they arrived, is slowly being revealed. The roves of the old town, too, are becoming visible. He races back to the settlement to tell his news.

Meanwhile, in the hospital, Martha is dreaming of slime creatures and yearning for water, and out at the old Sunday City the otters are breaking down the dams which have kept the buildings and space ship under water since the tidal wave.

Ty tells the Doctor that the ship was a mark II world-builder with a power core that would allow them to start up the settlement’s manufacturing system in time for the next wave of colonists. The Doctor is disgusted when he hears this: mark II ships were powered by dirty fission reactors. Ty says that the plan was to use a cheaper fission reactor until the colony was rich enough to replace it with a clean fusion model. Either way, the Doctor thinks the water levels are dropping for a reason and wants to investigate.

In the hospital Martha has drawn a sketch on her notes: a planet being gripped in the tentacles of some monstrous beast. Later she dreams that she has returned to the water. When she wakes up she finds that she is in the hospital with no memory of preceding events, and a vague idea that she should be in New York. She has no idea that the Doctor is feigning the identity of an adjudicator and all but gives the game away. When she is shown the picture that she drew she snarls and attacks the nurse and doctor at her bedside.

Chief Councillor Pallister is worried. The arrival of an adjudicator has set colonists thinking that there is an imminent evacuation due. In addition, Pallister’s election victory after the tidal wave was dubiously obtained.

Out in the wetlands the Doctor is speculating that the arrival of the slime monsters may be connected to the meteorite. When they arrive at the sea Ty, Orlo and the Doctor see hundreds of otters dismantling the dams. Deciding that this behavior is not natural Orlo and the Doctor try to capture one of the otters but only succeed when Ty shoots it with a tranquiliser dart. However, this causes the rest of the otters to chase them but the Doctor holds them up with a burst of noise from his sonic screwdriver. This allows the other two to escape and once he is sure that the otters are no longer in pursuit the Doctor makes his way back to the settlement. On the way he notices some other otters heading purposefully to the old settlement, which is now above water. One building in particular seems to be the target of the otters’ endeavours.

Candy is worried that Col has vanished. She picks up his trail at the edge of the settlement and follows him to the One Small Step. The ship is now out of the water but in a sorry state. She climbs into the dark hulk but when she reaches the flight deck she sees some of the consoles have been powered up. Col is lying, apparently comatose, on the floor. When she talks to him Candy realizes that Col is conscious but complaining that something is in his head trying to learn. He tries to warn Candy away and tells her to report back to the Doctor that the thing is after intelligence, flicking through his head like a book. It is then that she notices tendrils of a creeper attached to his head. Finally he asks her to, “tell them I’m sorry,” before pulling the creeper out of his head and dying.

When the Doctor gets back to the hospital he finds Martha restrained with leather straps. He notices that her charts resemble someone suffering from a massive allergic reaction and asks the doctor to give her a massive injection of adrenaline. On his way from the hospital he tries to avoid Pallister but the Chief Councillor tells him there is a council meeting that night and asks the Doctor if he can tell the other councilors why there is an adjudicator on the planet. The Doctor merely tells him, “Trouble.”

Trimming away the fur around their captured otter’s head the Doctor finds the same puncture wounds that he discovered on Martha. He extracts some cerebrospinal fluid for testing in the bio lab. He eventually discovers that the otters are subject to the introduction of RNA which is causing them to be aggressive and unintelligent. Their apparent improvement in intelligence when they reach the lab is actually a return to their former intelligence.

The Doctor decides that the only way to find out what the slime creatures are up to, and thereby help Martha over the effects of her ordeal, is to inject himself with their RNA. When Martha awakes in the hospital Dr. Hamshi tells her that the adrenaline and antihistamines have worked and she is much improved. As soon as he goes off to find her some food Martha is out of bed and making her way to the twilight outside. She finds herself in the town square, surrounded by long low wooden buildings. Then she sees dozens of otters swarming through the streets. As Martha heads one way to find and warn the Doctor the otters invade the council meeting. They stand on their hind legs to show their teeth and claws to the councilors, then acting on a single impulse they close in as if driven by a single thought.

In the bio lab Martha finds the Doctor strapped to a bed. His eyes open. They are totally dark with an oily sheen. He says, “All of you will…be…ME.” Beside the bed Ty Benson informs Martha that the Doctor instructed her to inject him with the proteins and RNA from the slime creature’s assault on Martha. Martha wonders if the Doctor’s alien physiology will make him more or less able to fight off the creature’s influences. Ty has just told Martha that the Doctor has two hearts when they hear a scream and a receptionist runs in to say that the otters have arrived.

They lock the doors and Ty produces the sonic screwdriver from the Doctor’s clothes, saying he used it before to fight off a previous otter attack. Martha uses it to drive off the otters in an attempt to get to the zoo lab and find some tranquiliser guns.

Soon after they notice that the Doctor’s fever is dropping. Almost immediately a messenger informs them that the otters have gone and so have twenty settlers, including Martha. They search the town and find evidence of struggle: scratch marks on wood, bloodstains in the council chamber and the sonic screwdriver trampled into the mud on the edge of the settlement.

A mud stained Candy tells Ty Benson about her trip to the One Small Step and Col’s death. On her way back to the city she saw swarms of otters leading the twenty or so settlers away, nipping at their heels, but did not see Martha among them.

In fact Martha had been avoiding the chaos of the capture of the settlers when she had tripped over some otters in the dark and dropped the screwdriver. However, these otters spoke to her, demanding she follow them to help them to help her. They led her to the darkness of the forest. In one of their nests they tell her that they have no leader, but that a Leader hurt them and wants to hurt the humans. They want to give her something to help stop the leader and produce a wicker sphere containing a small slime creature. At first she thinks it is a baby but the otters tell her it is a piece broken off from the other creature and that they want her to use it to kill the ‘parent’.

In the bio lab the Doctor has returned to his old self. He says he learned that the creature wants the humans for something and says he needs to get to the river where the prisoners were taken. He says that the slime creatures are using the information they have gathered from their victims and it relates to the technical services unit, the nerve centre of the original colony before the flood.

Out by the river the Doctor, Ty and Candy see the otters standing stationary while six settlers move among them like robots. The Doctor thinks the settlers have been injected with proteins that give them instructions to fetch and carry instruments and materials that the creature wants. The otters are set to guard them and make sure the orders are carried out. When some of the otters spot the Doctor and his friends they begin to advance, which is when the Doctor discovers that his screwdriver is full of mud. He sends Candy back to the city to tell what they have found, then he and Ty go down to meet the otters who part as if to let them through to the river.

On the way down the Doctor informs Ty of his theory about the slime creatures. He thinks they are rather simple and unintelligent, but when they arrive on a new planet they infect whatever creatures they find with their RNA and instructions to tell them about local conditions and circumstances. They use the intelligence of the natives to process the information and then upload whatever they need to survive and prosper.

At that point Pallister is raised from the water, supported by a huge tendril that pierces hi head in three places. The creature uses Pallister’s brain and mouth to tell the Doctor that it is going to use the humans to help it achieve its single goal: reproduction. It sends another tendril toward the Doctor saying that it intends him to be part of the creature as Pallister is. The Doctor uses one of Ty’s tranquiliser guns to shoot Pallister, rendering him useless for communication. The creature withdraws into the water, dragging Pallister with it. The otters around them are intent on keeping them penned in by the water until the creature returns until Martha and a dozen otters arrive from the top of the river bank, whooping and shouting. This breaks the guards’ concentration and conditioning and they flee.

Martha presents the wicker ball to the Doctor after he has woken up the handful of settlers from their hypnosis. He uses their return journey to the city to tell her that her friendly otters were recently in the zoo lab at the settlement and so had recovered from the creature’s conditioning and were acting for themselves. He needs to question the settlers who they rescued and investigate the creature in the ball.

The rescued settlers say that being under the creature’s control was like seeing the world through somebody else’s eyes and there was a sense of being violated but none of them can remember what they were doing. Then Martha suggests to the Doctor that he use his psychic paper while he interviews them somehow this draws out the answers he needs: the creature is building something.

He investigates the baby slime creature and realizes it is the end of a tentacle of the parent which the otters sliced off, but it is capable of growing into an adult itself. In the council chamber he explains his ideas to some of the settlers. He thinks the creature reproduces by breaking itself into smaller pieces. Because it can grow to almost unlimited size the only purpose for it to reproduce would be to colonise other planets. To do that it needs to travel through space, but because it is technologically unsophisticated the Doctor thinks it is going to use Pallister’s expertise in nuclear power to make a bomb and blast itself into space. It would probably be broken into several smaller chunks which would one day make planet fall on meteorites, just as it arrived on Sunday a few months earlier.

Looking for the remaining Sundayans takes Candy back to the space ship. She finds that Col had gone there to delete Pallister’s records. It seems that Col had supported Pallister’s election victory but had needed to suppress a history of petty crime including fraud and embezzlement. In fact, Col had managed to present Pallister’s record as that of a model public citizen when the election campaign was on. Was that why Col’s last words were to apologise? Candy then becomes aware of the sound of drilling in the distance.

The Doctor puts forward his theory that the creature is going to drill a shaft into the planet, drop an atom bomb and use the blast to surf into space just as Orlo arrives to say that the drilling has started.

The Doctor returns to the bio lab and creates a new synthesis of the proteins which he injects into himself. Later, when Martha finds him he still seems his normal self but he is filling tranquiliser gun cartridges with the fluid.

Orlo is sent out to spy on the drilling operation. He sees the captured settlers drilling the shaft, but by the drill control room he sees Candy. She pretends to be one of the controlled colonists and simply walks past the others and the otters into the control room.Looking out of the window she sees what seems to be a huge bomb being taken to the lip of the shaft. From it a long cable extends back to the control panel near her in the room. She tries to unplug it but it is locked in place. Then she is surrounded by otters.

The doctor and Martha arrive at the drill site just in time to see the bomb tip into the shaft. They realize they are too late and about to die. Then they see the cable from the bomb following it into the shaft as if someone forgot to plug it in. Orlo tells them it must have been Candy’s work because she comes running from the control centre at that moment, accompanied by the otters. She says that the otters who found her had talked to her and she had told them to chew through the cable.

As they make their way to the drill to dismantle it the creature raises up the decaying body of Pallister to talk to them and tell them they would interfere no more. The Doctor offers the creature one last chance, saying he will use the TARDIS to take it where it wants to go. The creature uses its tendrils to engulf the Doctor’s head, saying it will use him to control the TARDIS for it to go wherever it chooses. Ty fires one of the tranquiliser darts at the creature but it merely bounces off the thick hide. With a second dart she shoots the Doctor.

The creature’s skin takes on a frosty texture, spreading out from where it holds the Doctor’s head. The alien’s body fluids begin to pump out of it and it drops the Doctor as it enters its death throes..

Later, the Doctor tells Martha that he concocted a solution that would attack the alien’s metabolism and that was what he had put into the cartridge that Ty fired into him as the creature dragged him away. He couldn’t have injected himself earlier in case the creature detected the solution within his body.

He also says that the creature must have found out about Pallister’s nuclear know-how from Col: the Chief Councillor must have been on Col’s mind when he was attacked while he was erasing the records on the ship. Once it had Pallister the creature had the understanding to order the settlers to concoct a nuclear bomb.

Ty can’t understand why the otters have suddenly started talking but the Doctor says they probably had their own language but it only became intelligible to humans once the TARDIS arrived.