Fra’ Vallir enters the council hall on Karagula. The other eleven councillors are already present, as is a young, beautiful, dark-haired woman. Vallir is nervous. This woman has recently used powers beyond his comprehension to give his people their greatest desires, including returning dead relatives to life. She has come to claim her reward. The councillors are appalled when she says that she wants all of their children.

The Doctor steps out of the TARDIS under Karagula’s twin suns. Even for his metabolism it is too hot and he casually tosses his jacket into the TARDIs before setting out to survey the barren landscape. He is suddenly surrounded by five humanoids, all pointing spears at his throat.

Bethin regains consciousness and finds that she is walking across the desert. The heat is almost unbearable. She sees the other children of the village walking in step around her. Their expressions are blank and they seem to be sleepwalking. As she stumbles to a halt the beautiful woman approaches. For a moment Bethin’s eyes seem to trick her; instead of the woman she sees a multi-limbed reptile. The vision passes and the woman trails a dark vapour under Bethin’s nose. Before she slips into unconsciousness again she is aware of a huge building in front of her. Although she has never seen it she knows it must be the Dark Cathedral that fills her elders with dread. Like an automaton she begins to walk again.

The Doctor is taken down to a settlement. It is made up of dwellings that lie under huge canopies, like sails, that ward off some of the heat. He notices that the houses are all made of reclaimed materials; it is a ramshackle shanty town. More intriguingly, each building stands on a raised platform that hovers above the ground. The buildings are all coupled together like a train and are moving, albeit very slowly. Larger platforms intersperse the village and these carry agricultural plots. The villagers stare at him from their platforms and their faces are distinctly hostile. The Doctor is surprised not to see any children.

He is taken to the hut belonging to the head man who tells the Doctor that his arrival in the TARDIS was observed. He is accused of being a trespasser and as a stranger is unwelcome. The Doctor promises that he has no sinister intent but the head man tells him that their naivety has cost them dear recently and until he has told his business to the council the Doctor must be locked up.

High Minister Drakon watches the children file into the Dark Cathedral. The beautiful woman in blue robes leads them into the depths of the building. He finds himself disapproving of her Dreamspinning but he knows that some of his Brothers and Sisters, even members of the ruling Witan, are partaking of her concoctions. His thoughts are interrupted by news that the Agent has returned to Karagula, bringing the Eternity Crystal.

The Doctor’s cell is a basic affair. In a similar cell is an old man who is fast asleep. A young boy comes to visit the old man but the Doctor engages him in conversation first. The boy’s name is Lorton and he tells the Doctor that the head man, Fra’ Vallir sees all visitors as a threat since the travelling woman took all the children. As an aside he adds that his grandfather was locked up after he began screaming and yelling that the woman was really a monster and tried to attack her. The woman called herself a ‘Dreamspinner’ and granted the villagers their hearts’ desires. She spent three days asking everyone what they truly wanted and on the fourth day she gave them what they asked for. In return she took all of the children, including Lorton’s sister Bethin. Lorton didn’t go because after his grandfather was locked up he ran away into the desert and when he was fetched back he had the Sun Fever and was too ill to walk with the others. The Doctor offers to help get the children back but Lorton has to let him out of the cell.

The Doctor returns to Fra’ Vallir’s hut and introduces himself. He offers to get the children back but the Chief Councillor tells him they swore the Lithic Oath and that cannot be broken. The Doctor suggests that the woman duped them and that the oath is not valid. He asks for examples of the miracles that the woman performed for the villagers. When he is told that Fra’ Sagral’s wife was returned from the dead the Doctor questions deeper and Fra’ Vallir realises that he can remember nothing of the woman’s funeral. The Doctor puts it to him that the woman spent the first three days hypnotizing people into thinking that they had lost relatives or limbs so that on the fourth day it seemed that these things had been restored. Fra’ Vallir looks more hopeful but says that the Oath is enforceable by the Dreadbringers of the Darksmith Collective. This raises the Doctor’s interest and he questions the Councillor about the Darksmiths.

He is told that the original colonists of Karagula found the heat of the twin suns unbearable. They split into two factions: one group migrated across the planet under their sun canopies while the Darksmiths retreated underground. The Darksmiths developed their technologies and traded with alien races via the Dark Cathedral, their only portal to the surface. The surface dwellers are wary of upsetting the Darksmiths because their own culture depends on living off the scrap heaps of their more advanced cousins. The Doctor decides to pay a visit to the cathedral once he has collected some items from the TARDIS. Fra’ Vallir tells him that the blue box has been brought into the village.

The Doctor makes his way across barren rocky terrain, oblivious to the fact that Lorton is following him. When a giant Rock spoorl plunges from the sky and attacks the Doctor it is down to the young boy to kill it expertly with a rock from his sling. The Doctor looks at the spoorl’s corpse and comments on its unpleasant stench. Lorton remarks that it wouldn’t bother his grandfather who has lost his sense of smell. As a thankyou for saving his life the Doctor lets Lorton accompany him towards the huge shape of the Dark Cathedral. As they approach the edifice the Doctor spots a square rock and says it is the door to an exit tunnel that Vallir told him about.

In the cathedral, Drakon watches technicians working on the Agent. Brother Ardos tells him that they have recovered the Crystal but the Agent was so badly damaged that they are having to go into the giant robot’s data core to find out what happened to Brother Varlos. They are interrupted by Sister Hellan who tells them that a ‘John Smith’ has arrived with a business proposal.

The interior of the Cathedral is pitch black. The Doctor has sent Lorton to scout out the interior and locate the missing children. He makes his way down tunnels and passages until the scent of orange blossom leads him towards his goal. The last time he smelled it was when the Dreamspinner came to the village.

In the chamber of the Witan the Doctor uses the blue light from his screwdriver to find his way. Even its meager light is almost blinding to the nine Darksmiths gathered there. Drakon tells the Doctor that they despise any form of light. Power and creation come from darkness, he adds. The Doctor responds by telling them that he has come from the Spiel Confederation in the Blarney Cluster and needs their help to revive the extinct Twaddle population on which their economy was built. He says that his people have heard that the Darksmiths invented a device that reanimated dead matter and that a man called Varlos was key to its construction. Drakon coldly replies that Varlos has left the Collective and that the device was unsuccessful. Moving smoothly on the Doctor asks if he can inspect the Darksmiths’ catalogues and see if they have any other devices which his people could buy. Brother Jaxal is instructed to take the Doctor to the library.

Lorton finds himself in a square stone room. In one corner is a large creature, part reptile, part insect, with ruined eyes. It says that it can smell his fear and to calm him it changes its appearance to the beautiful woman who took the children from the village. Lorton demands to know where his sister is but the creature releases black snakes from the walls and Lorton screams in fear.

The Doctor is reading quickly through the Darksmiths volumes of the jobs undertaken by them over the centuries. None mention either the Eternity Crystal or Brother Varlos. He uses the sonic screwdriver to render Brother Jaxos unconscious before investigating a secret door. Inside a small room he finds a book. Using the screwdriver to access the book’s data port he uploads Astrule Varlos’s development journal to the TARDIS. Then he hears a boy screaming. He races down the corridors while placing a clothes peg over his nose and arrives at the chamber where Lorton is facing the snakes. He tells Lorton to hold his nose then turns on the monster. She identifies herself as Shas-Raklat, one of the Menim. The Doctor is puzzled because the Menim are renowned for their eyesight but this creature says her eyes were put out by her own kind as a punishment. She uses her sense of smell now and synthesizes odours to alter memories and create visions. The Doctor derisively points out that all he and Lorton had to do to overcome her powers was block their noses. Shas-Raklat promptly advances on them, wielding the spiked tip of her tail.

Lorton and the Doctor flee up the corridors. As they go the Doctor explains how the Menim creature used her scents to fool the villagers into thinking that their dreams had come true. Only Lorton’s grandfather, who could not smell her odours, saw through her disguise. Worse, some of her modifications have been made with alien tissue and he fears that she will use the missing children to add to her powers.

The Agent recovers its consciousness and immediately swings into action, detecting Lorton and the Doctor in the cathedral. The two intruders have found Shas-Raklat’s spaceship parked in a hangar. They enter the craft and the boy sets off to find his sister while the Doctor finds a phial that interests him in the Dreamspinner’s store cupboard. A shout from Lorton takes the Doctor to a room where the children are wrapped in cocoons. The Doctor shows the boy how to revive them and then tells him to lead them back to the village. Then he hurries off to find the Crystal but Lorton follows him. He has only gone a few steps into the corridor when he is confronted by Drakon and several uniformed guards as well as the Agent.

Drakon holds out the Crystal and tells the Doctor that it rightfully belongs to the Darksmiths. The Doctor argues that it is dangerous and he only wants to remove its threat from the Universe. He whispers to Lorton that he wants him to take out the Agent with his slingshot. He throws the Dreamspinner’s phial at the Darksmiths where it smashes. Immediately, they begin to scream in terror and run away. Lorton flings a stone at the Agent, hitting its control panel and causing its rocket thrusters to ignite. Drakon, who had the presence of mind to block his nose when the phial smashed, is knocked over and drops the Crystal. The Doctor snatches it up as the robot brings its guns to bear on him. Just then, Bethin unleashes a blast of laser fire from the Menim ship that knocks the Agent over and lets her brother and the Doctor escape into the ship. The Doctor orders Bethin to blast an opening for them and then powers up the engines and flies them out into the clear skies.

They land beside the village and the children race out to their parents. The Doctor returns to the TARDIS and stows the Eternity Crystal in the stasis box before looking at the data he uploaded from Varlos’s book. He notes that the last page contains co-ordinates for a planet called Flydon Maxima and decides that he needs to go there to try to track Varlos down. He spends a short time saying his farewells to Bethin and Lorton as well as reminding them that Shas-Raklat’s ship has enough technology for them to liberate themselves from the Darksmiths’ leftovers.