Sister Serenta has taken a break from digging an animal trap with her brother Purin and his friend Aerack. She is instead gathering fruit in one of Laylora's forests. As she makes her way back to the boys she hears a loud crashing sound in the trees and is pursued by a large hairy beast with four arms and lethal talons. She flees to the beach where she is finally caught. She realises that the beast in front of her is one of the Witiku of legend before it attacks her and she loses consciousness.

Aboard the TARDIS, the Doctor and Rose are playing pool (with balls designed to resemble the planets of the solar system) before they arrive at their next destination, which the Doctor claims has been selected by Rose's MP3 player set on random shuffle.

Back on Laylora, the Tribe of the Three Valleys are concerned about the disappearance of their three youths. Most of the men are away moving animals to winter grazing lands and Mother Jaelette is in charge. At a loss for what to do, she hopes that the ritual of the shaman, Brother Hugan, will provide divine intervention.

A very old spaceship drops out of hyperspace. Grimy and patched-up, it is the SS Humphrey Bogart and it is owned by Professor Petra Shulough. With a makeshift crew of four, Shulough is eighteen months into an exploration voyage in search of The Paradise Planet. She is accompanied by an experienced old marine, Kendle, who is in his late sixties. The other three are very young: Jae Collins is a playboy pilot fleeing his gambling debts; and just out of the Space Naval Academy are Ania Baker and Jonn Hespell.

On Laylora, young Kaylen remembers the moment fifteen years earlier, when she was six, that she heard an ear-splitting roar above the forest and saw a plume of black smoke cross the sky. A dark object fell into the trees. When Kaylen reached it she found a small egg-shaped craft with a baby boy strapped inside. This child was adopted by the tribe and raised as her brother, Rez. Kaylen and her brother Rez are now assisting Hugan the shaman in one of his rituals, and Kaylen thinks that the shaman looks more silly than imposing in his ceremonial robes. His rituals and ceremonial costume reflect a time when the Laylorans had worshipped their world as a goddess and their religion had included blood sacrifice. Modern inhabitants of the world, still living in harmony with their environment, are less inclined to see the planet as a living deity. However, the trio are performing a rite for the three missing youths when there is a sudden earth tremor, shaking the ancient temple where they stand and causing its walls to collapse.

The Humphrey Bogart, approaching the planet, is suddenly struck by an electromagnetic pulse that disables most of the systems. Major Kendle, the old and experienced pilot, uses manual control to effect an astonishing crash landing. As he does so, Hespell launches a space buoy sending a distress signal.

The TARDIS receives the signal and the Doctor diverts to Laylora. Rose steps out and is amazed by the beauty of the forest, mountains and beach. In the distance is a plume of black smoke, indicating the site of the crashed spaceship.

Inside the spaceship the five crew members are bruised and shaken but still alive. Damage is minimal but the power is down and forty-eight hours are needed before a take-off can be attempted. Professor Shulough and Major Kendle rig up a back-up power supply in the cargo bay to get the main computers working. As Shulough works at her computer, Kendle comments on how well the young crew handled the emergency. The Professor barely responds because she has confirmed to herself that she has achieved a goal she has been working towards for many years: she has found the Paradise Planet and they are on Laylora.

On their way to the crash site Rose and the Doctor discover the sprawling ruins of the temple and spot the recent effects of the earthquake. The Doctor is fascinated by the crude paintings on the walls of the temple, particularly those of monsters with four arms. Realising he has lost his bearings he climbs the tall central tower of the temple, leaving Rose at the bottom. She hears a noise coming from one of the nearby outbuildings, goes in to investigate and is confronted by a monster with four arms and sharp talons. Rose rolls from side to side to avoid the monster but instead of being attacked she realises it is a young man in a costume. He is blond, young and attractive. And human. She tells him her name and he introduces himself as Rez.

Kendle and Hespell are checking the outer hull of the spaceship for damage. The younger man sees a massive creature, covered in thick black hair and with four arms, emerge from the forest. The two men race back to the airlock and shut the door. Back on the bridge they look at the screens just in time to see one of the monsters destroy the security camera with one blow. Afraid that the monster's talons can slice through the hull, Kendle and Hespell connect the generator to the metal outer plates. The three monsters on the spaceship are hurled away and disappear into the forest. Kendle prepares to go after the monsters and orders Hespell and Collins to follow him. Their journey takes them to the temple ruins, where Hespell fires a shot at a movement on the roof and hits the Doctor, who was on his way down from the tower.

Rose and Rez witness the shooting but remain silent because of the men's weapons and uniforms. However, as the Doctor is picked up and taken away, he spots Rose and winks at her. She is keen to follow him, but Rez says that the forest is dangerous at night and that she must go with him to the village until morning. The tented village reminds Rose of a Native American settlement. The villagers want to know if she is from the crashed spaceship. When a beautiful girl embraces Rez, Rose feels a pang of jealousy and is delighted to find out that she is his adopted sister Kaylen. Kaylen recognises that Rose and Rez are similar. Rose notices that the Laylorans have only four fingers, among other slight differences to the human norm. Kaylen explains that Rez was found as a baby, and Rez tells Rose that he does not think of the Tribe as anything but his true family. Brother Hugan then appears, announcing that the Witiku will be called forth, and Rez explains that they are the monsters Rose saw in the temple paintings and that he was dressed as one when they first met. Hugan is apparently attempting to whip up a frenzy of bloodlust so that Rose can be sacrificed, but as soon as Rez objects the frenzy dissipates and the old man apologises, saying that was not his intention at all.

Hespell is finding the job of guarding the 'unconscious' Doctor tedious. The Time Lord pretends to wake up and quickly wrong-foots the youth with a barrage of questions and some cheerful banter. Hespell soon finds himself talking about the attack by the monsters, much to the Doctor's interest. When Kendle intervenes with the intention of interrogating the Doctor, he finds the Time Lord keen to talk, but after two hours of conversation he is no nearer to discovering who the Doctor is. Professor Shulough decides to take a turn. She is surprised when the Doctor mentions that he has no idea which planet they are on. When she tells him that it is called Laylora, he says that he thought it was only a myth, though he is uncertain if he heard about it in the past or some time in the future.

She takes him to her quarters and shows him the handwritten journal of Maurit Guillan, an explorer on a survey mission for a big corporation. Guillan's ship, the SS Armstrong, was discovered drifting in space. Its crew were dead, power and life support were lost and the ship had been stripped by pirates. Scans revealed that the hold had been full of huge amounts of trisilicate, a rare energy source, incredibly valuable. Nobody could work out where Guillan had been except for some pictures in his cabin labelled 'Laylora' and 'paradise'. This led to the legend of 'The Paradise Planet'. For a while people searched for it, then interest faded. Only Shulough carried on with the search. After finding the journal she became obsessed with the desire to find the mysterious planet.

Their conversation is interrupted by the smell of burning. The Doctor rushes to a locked room and forces the door. Inside the room a short circuit has started a fire and noxious smoke has overcome crewman Collins. The Doctor puts out the fire. When he discovers the young man is dead he is surprised by the lack of reaction from the Professor. She comments that the young man's duties were not integral to the success of the mission. Nonetheless, the Doctor offers to help restore some of the ship's systems and the Professor accepts.

Rez and Rose spend the evening in his tent, talking and eating. He explains to her that Laylora is a planet of balance and harmony but that recently a series of events more bad than good have spoiled the balance: failed harvests; strange weather; earthquakes. He tells her that Hugan thinks the planet will summon the Witiku to protect itself. He offers her a drink of jinnera, a coffee-like beverage that relaxes the drinker but can kill if overused. Rez tells Rose that he believes the Witiku were once real but that they no longer exist. At that moment a Witiku rips away the wall of the tent.

The Witiku is a huge creature covered in black fur, smelling strongly of sweat. It reminds Rose of a werewolf with red eyes and fangs, but it is wearing something that glitters around its neck. Jumping past it she finds her escape cut off by another Witiku. Her only weapon is a hot cup of jinnera, which she throws at the creature and which, to her surprise, causes it to sink to its knees and scrape off the liquid. Rez pulls Rose from the tent and into the panic of the village. It is impossible to see how many creatures are attacking. Following Brother Hugan's advice, they escape towards the temple, stopping only to see a Witiku carry off a villager. They meet Mother Jaelette in the forest, and she reveals that only three monsters attacked the village but they can see about a dozen on the way to the spaceship. Jaelette also tells them that Hugan is missing.

The Doctor is busy repairing the spaceship. Hespell is perplexed when the Doctor claims to be older even than Kendle. The Doctor demands to see the emergency generator and is horrified to find that it is a micro-fusion generator that is operating in the ship's cargo bay. Such devices are banned on most civilised planets. This one is venting its exhaust directly into the air around the ship. He finds Shulough and Kendle to berate them about this when the Witiku begin another attack.

Kendle breaks out the small arms and the four surviving crew members take their guns and step out of the ship. The Doctor refuses to carry a weapon and when two Witiku drop from the hull in front of Professor Shulough he prevents her from shooting them. Instead he tells her to let the creatures go into the ship so that he can find out what they want. The Witiku make for the cargo bay and destroy the emergency generator before returning to the forest.

Outside, there has been a fierce fight. The female crew member, Baker, has a wounded shoulder and one of the monsters is unconscious. Kendle and Hespell carry it into the ship while the Doctor tends to Baker's injuries.

Rose wakes up in the temple complex, where she spent the night with the villagers. Entering the main temple she finds a stone altar stained with old blood. Downstairs in the crypt she finds stores of grain and jinnera beans as well as older paintings of the Witiku. One room is piled high with crystals. She is discovered there by Kaylen, who tells her she can take as many crystals as she likes; they are ubiquitous on the planet. Kaylen also tells her that eight villagers went missing in the night attack, including Hugan.

When Rose returns to the surface she hears that some of the Laylorans are planning an attack on the spaceship. Rose points out that the Witiku are not from the ship, which has a human crew. She offers to go to the ship herself to ask for help. Rez says he will take her.

On the ship, Hespell checks on Baker and finds her condition improving. She thanks him for saving her life. He is embarrassed that there seems to be a warmth between them. He goes to find the Doctor examining the Witiku chained in the cargo bay. The Doctor discusses the creature with Hespell, saying that he wants to take the chains off because he thinks the creature is harmless, but Hespell refuses to disobey an order. The Doctor says that Baker was only attacked after she opened fire; the attacking monsters left after they wrecked the generator. The Doctor thinks that their attack as a group shows a primitive quality but their knowledge of what to destroy reveals a higher intelligence. He is also drawn to the necklace that the Witiku is wearing, which contains gems made of trisilicate.

When Rose arrives there is an emotional reunion with the Doctor. Rose introduces Rez as an orphan who fell from the skies and there is the first sign of emotion from Professor Shulough. Rose explains to the Doctor how jinnera seemed to be a weapon against the Witiku. Shulough questions Rez about being an alien orphan and the Doctor asks the young man to show him a jinnen plant. The Doctor takes some samples aboard the ship to analyse.

Ania Baker finds the Doctor in the laboratory. He has made a jinnen solution to use against the creatures and the two of them set off to the cargo bay to get a cell sample for some tests. Meanwhile, Rez gets bored and wanders off to the cargo hold where he finds Hespell playing a video game. Rez has a go at the game and masters it immediately. Neither pays much attention to the chained Witiku. Rose enters the bay and finds the two young men absorbed in their game. She inspects the unconscious Witiku and is surprised to see it wearing a necklace that she recognises. To her horror it is feigning unconsciousness and leaps at her, ripping its chains from the wall. She is knocked against Hespell who drops his weapon.

At this point the Doctor and Baker arrive. Seeing the danger Rose is in the Doctor throws his jinnen solution at the beast. It immediately transforms into the shape of Brother Hugan. Rose realises that the monsters are, in fact, the missing villagers.

Hugan is taken to the MedLab for treatment. Shulough announces that his vital signs are normal. The Doctor decides to make more jinnera, and Rez says he will take them to the village where there are huge stockpiles of jinnen. As Shulough, the Doctor and Rose plan their trip to the village, Rez blunders in suffering from a head wound. He says that Hugan attacked him and fled from the ship. The professor tends his injuries, again showing more sympathy than was evident earlier when her own crewman died. Outside, Hugan runs like a man possessed, ready to rouse his people in a battle to remove the aliens and their technology from his world.

The Doctor finds traces of Hugan's trail as the party make their trek to the village. He tries to involve Professor Shulough in a conversation as he attempts to work out her motivation for discovering the whereabouts of the Paradise Planet but she is less than forthcoming. When they get to the village the members of the Tribe have already returned from the temple and resumed their life. Hugan is back in his tent and Kaylen takes the Doctor and Rez to speak with him. The Professor is amazed by the jewellery of the Tribe, made from perfect trisilicate crystals. Rose offers to take her to the temple where there is a room full of them.

Hugan's speech is incoherent and rambling, returning always for the need for Laylora to be cleansed. Kaylen gives a cup of jinnera to Hugan but he spits it out and leaps from his bed, racing out of his tent past the Doctor and off in the direction of the temple. The Doctor instructs Rose and the professor to follow Hugan while he stays behind to get more jinnen. Kaylen and Rez help him to carry bags of the seeds to the spaceship. Hespell and Baker greet them and the Doctor is pleased to see the growing intimacy between the two crew members.

The Doctor leaves Hespell and Baker concocting more jinnen solution and works beside Kendle creating a delivery system that can fire the jinnera at the Witiku. The Doctor discovers that Kendle is Shulough's uncle. His sister and brother-in-law, Shulough's mother and father, were part of Guillan's crew and died on the Armstrong on its return from Laylora. Kendle says that the death of her parents changed Petra Shulough from a happy ten-year-old into an emotionally bereft and fiercely driven academic, following a life and career that led to Laylora.

Night falls as Rose and the professor try to find the temple. The professor insists they leave the path, contrary to instructions from the villagers, to take a short cut. She promptly falls into a pit dug as an animal trap. While Rose is extemporising a rope made from some creepers, a Witiku arrives on the scene. Rose has to undertake a complicated series of manoeuvres but ends up with the Witiku in the pit and the professor out of it. The two women race off to the temple ruins but when they reach the walls they find themselves surrounded by Witiku.

Kendle allows the Doctor to read Guillan's journal in the professor's quarters. The Doctor realises that the previous expedition made a hasty departure from the planet when they deduced that their mere presence was destroying the balance of the planet's ecosystem. He tells Kendle that the planet is allergic to alien intrusion and that the Witiku are the response to new elements.

Meanwhile, Rose and Shulough climb the uneven temple wall and enter through a window. They go down to the crypt, where a maze of cellars might offer escape; also, that is where the crystals are. They eventually arrive at a huge chamber surrounded by five or six enormous statues of Witiku. Realising they are in a killing pit where sacrifices were once made they pass through an open archway, not realising they are being followed.

Kendle leads Baker, Hespell, Rez and the Doctor from the ship. Each is wearing a belt from which hang bags of jinnen solution and each has a pump-action water pistol connected to a tank of the liquid. Baker and Hespell hold hands as the party traverses the night-time forest. The Doctor talks to Rez who sadly informs him that Laylora was once a perfect planet but is now blighted by crop failures and unusual weather patterns. The situation is worsening year by year and Rez claims it only started to go wrong when he arrived and was found by Kaylen.

Witiku force Rose and Professor Shulough to take refuge in the chamber of crystals and the two women resort to throwing handfuls of trisilicate at the monsters to keep them at bay. Hearing the noise the Doctor and his party race down to the crypt and deploy their weapons, soaking the Witiku but otherwise having no effect other than to anger them. As the monsters turn to attack the newcomers, Rez, Hespell and Baker race back into the corridor. The Doctor and Kendle run around the Witiku and join the women on top of the crystals where the Doctor sees a trapdoor above their heads. Rose climbs on his shoulders to open it and the two women scramble up to the room above. Brother Hugan is waiting for them, dressed in the Witiku costume. He takes the professor hostage and places the talons at her throat while ordering Rose to shut the trapdoor.

Hespell, Rez and Baker decide to return to the Doctor and Kendle when a party of villagers arrive in the temple. The young humans explain that the jinnen solution was not strong enough. Jaelette, at the head of the villagers shows them the weapons they have brought, sharpened spears made from the jinnen bush tipped with jinnen paste. They reach the chamber while Kendle is shooting the crystals from under the feet of Witiku trying to climb the mound. The villagers hurl their spears or jab at the Witiku who transform immediately back into villagers. Jaelette's party have brought blankets and clothes to cover the nakedness of the missing villagers. The Doctor thanks Jaelette who tells him that every missing villager has been accounted for except Brother Hugan. She also informs him that the room above is the Hall of Offering where sacrifices were once made.

Hugan forces Rose to tie up the professor at the feet of one of the giant statues. He knocks Rose out with a pad soaked in jinnera and places her on the altar. He begins a ceremony which will end with him plunging a knife into Rose's heart. The Doctor arrives and tries to distract the shaman by saying that all the new arrivals will leave the planet. Others creep into the room behind him and Rez tries to make his way round behind the statues. Hugan does not agree with, or believe, the Doctor's words so the Doctor offers himself as a sacrifice instead. He says he is the last of his race and has two hearts. This gives the shaman pause for thought, which allows Kendle to untie the professor. She immediately steps forward and offers herself as a sacrifice, too. Then Kendle offers himself, as do Hespell and the Layloran villagers. Hugan is confused and decides to simply stab Rose anyway. She has woken up and swings away from the blade even as the Doctor pulls her to safety. Rez has got himself stuck behind the statue. As he forces himself free it unbalances and crashes down, shattering into pieces. There is a moment of celebration interrupted by an earthquake.

Everyone except Hugan escapes from the chamber but in the confusion of the collapsing temple Kendle, Rez and Rose are separated from the others. Finding the way blocked Rez takes them down through the warren of tunnels and up a long flight of stairs up the tower above the temple. Brother Hugan transforms into a Witiku and gives chase. Outside, the Doctor sees that most of the temple has been destroyed. Through his night vision binoculars he watches the three humans being chased up the stairs. Pulling a handful of trisilicate crystals from his pocket he asks if it is enough to get the spaceship into the air.

The trio reach the observation deck at the top of the tower while Kendle holds the Witiku at bay with laser shots into the walls further down the stairs. The Doctor takes the controls of the spaceship and flies it to the temple, hovering just above the tower. Rose and Rez manage to leap aboard, pulled in by Hespell and the professor, but the transformed Brother Hugan lunges past Kendle to reach the ship. A laser blast to the back merely knocks the monster over and several more shots barely halt its progress towards the old marine. Kendle leaps forward to punch the Witiku in the face and both of them fall to their deaths.

A fierce storm prevents the Humphrey Bogart from leaving the planet. Rez admits that he knows he must leave. All the catastrophes, including this storm and the electromagnetic pulse that brought the spaceship down, have been caused by his presence. As the rain abates, Rose, the Doctor and Rez go to the ship to say their farewells. Rose congratulates Hespell and Baker on becoming a couple. The Doctor finds the professor in her quarters and commiserates with her over the death of her uncle. They agree that Laylora must be taken off the map. The professor says she will take Rez with her and try to be a mother to him.

Rez says his farewells to his family and the spaceship lifts off and gracefully departs. He takes with him a cube that was found in his escape pod and Professor Shulough shows him how it works. He sees a hologram of his parents, talking to him as they put him into the pod. For the first time he will know who he is and where he comes from. Professor Shulough realises she has a new, and more valuable, quest in front of her.