Karl and Katy Baring are a brother and sister out in an old dead wood at night. Karl is trying to scare his sister by suggesting that something will come to get them. They are looking at scientific equipment that has been damaged, but they don't know what is causing it. They see the old bridge down below them and a smooth trail of earth leading towards it as though something heavy has been dragged down. Karl goes down to investigate. Katy follows but when she gets there her brother is nowhere to be seen. She stands in the old dried river. She looks at the old bridge carrying rusty railway lines, long fallen into disuse. Thinking Karl is messing about Katy grows angry. When he emerges from under the bridge she shines her torch on him, only to discover that it is someone, or something, else. She turns and runs in fear. Crossing the dark ground in panic she collides with the Doctor.

As the sun rises the Doctor sees the landscape; parched hills and dead trees. He thinks that this is drought country but rain begins to fall, disproving that idea. He looks at the pretty blonde at his feet. She has been silent since he met her. He picks her up and leads her by the hand through a brown landscape. It is divided by plastic into fields but nothing is growing. He is sure it is England but cannot understand the devastation.

Petra Lancaster sits in the study at the Grange and watches an appeal for money which was broadcast on the internet by Karl Baring. Dr Timothy Hill is one of the volunteers at The Grange helping the Barings in their work. Petra tells him that the appeal has had 300 million hits; well below the target. Out of the study window they see the Doctor carrying Katy up the gravel track to the stately home. Petra rushes out to meet him. The Doctor shows her Katy's unconscious body and says she has collapsed on her way home. He shows Petra his psychic paper. It appears to be a document giving him unlimited clearance to investigate what is going on. Petra is mystified and the Doctor, in fact clueless about everything, tries to maintain an air of mystery.

Katy is taken to the infirmary and attaches the Automated Medical Unit (AMU). The Doctor begins to ask why England is so barren but is cut off when several more volunteers arrive. Charlie Bannon tells them that Karl and Katy's 4x4 is missing. Vanessa Foster arrives, too as Katy wakes and says, "Under the bridge."

Petra contacts Madrid where Hamilton Bryant, the food minister, reassures her that Karl will come back soon. Bryant says that a lot of people are gleeful at the failure of the Grange to grow any food. These new events will only add to their delight. He urges her to wait until the situation resolves itself. Petra distrusts him, seeing only a politician eager to distance himself from any failure.

Another 4x4 drives over the barren Hampshire fields. Charlie is driving the Doctor back to the bridge while Charlie fills him in on the global cooling that started a new ice age that poisoned the temperate zones. Karl Baring was running The Grange, an experimental agricultural complex that was trying to bring life back to the countryside of England. Britain is now deserted, its former inhabitants dispersed around the world, unable to return to their poisoned land. Charlie goes on to explain how synthetic food became the new oil, how the world divided in defence of its resources, and how that fragmentation has led to global mistrust and war. Karl Baring was the first to plough his billions into trying to restart agriculture in England but failure after failure has dwindled his resources until he was forced to turn to charity to maintain his project at The Grange.

They park by the bridge where Karl and Katy's car stands empty. The Doctor runs down to look at the bridge while Charlie follows uncertainly. He reflects on how cut off they are in England, a country isolated from the rest of the world and almost completely deserted. He takes his shotgun with him down to the bridge. The Doctor is peering at the mud and wondering why there is no sign of the previous night's events. There are no footprints or scuffle marks. The Doctor decides to go and talk to Katy. As the Doctor runs back up to the car Charlie notices something caught in the brickwork of the bridge and goes to investigate. When the Doctor returns a minute later Charlie has gone, leaving only his shotgun. Two hours later he is still searching for him.

At a poolside party in Spain Hamilton Bryant speaks to Petra over the relay link, telling her he won't be authorizing a rescue mission, even though two people have now disappeared from The Grange. Feeling lonely and isolated she cuts him off angrily. She calls a meeting of the volunteers in the common room and informs them that the Sat-Trak office in Paris no traffic has entered or left Britain in the last six weeks, so whoever took the two missing men has been there at least that long. She adds that she asked Bryant for help but doesn't think any is coming.

The burly South African, Campbell, blames the Doctor; his arrival has coincided with the disappearance and he adds that the Doctor's rescue of Katy was a ruse to get them all on his side. Petra disagrees. Her feeling is that any agro-spy among them is likely to be one of the volunteers. This strikes a chord with them and they look at each other suspiciously. The tension is undercut by the Doctor, who has been lurking at the back of the meeting, when he asks about the white fibers in the carpet. They all look down with surprise, wondering why they haven't noticed them before. The Doctor speculates that they are roots and Tim looks at them more closely. He says they are not part of the foods they have been either eating or attempting to grow.

The Doctor persuades Petra to let him speak to Katy and both of them enter the nursery which is where Katy is hooked up to the AMU under Vanessa's care. At first Katy only asks them to leave the light on and asks for Karl before whispering that she 'knows' and adds: 'not under the bridge'. They are interrupted by a call from Sanders who tells them tha5rt the relay link has gone down and they are completely cut off from the outside world. This can only be a result of sabotage somewhere in the grounds, unless the central office in Madrid has cut them off. The Doctor wonders if Katy's fear is something that has been there all along, a childhood terror that has been lying in her subconscious mind. Vanessa asks if the book on the shelf is relevant, pointing to a children's story called 'Not under the Bridge'. The Doctor snatches it up, reads it quickly and runs from the room.

He finds Tim analyzing the fibers of plant material from the carpet. Tim says he doesn't recognise the genotype; the proteins have been reprogrammed. The Doctor speculates that something is reshaping plant matter to make it walk around The Grange.

Back in the common room the Doctor has lit a roaring fire and made a meal of cheese sandwiches and hot chocolate for the volunteers. All are present except Katy, but her laptop is linked to the screen in the corner so that they can keep an eye on her in the nursery. The Doctor reads (or rather, gives a précis of) 'Not under the Bridge' a story about a troll that lives under a bridge and only wakes when he is hungry. The Doctor closes the book and tells them that they have got a troll on the premises. He takes their silence for rapt attention until he realises they are looking beyond him towards the doorway.

A small creature, like a woodlouse in shape but the size of a small baby, is crawling across the ceiling. The Doctor is much taken with admiration of the creature, which seems to be made of twigs. The volunteers begin to panic and scream. Campbell tries to knock the creature from the ceiling with a broom handle but it runs down the pole towards him. Before it reaches him it leaps into Vanessa's hair. She faints but the Doctor picks the creature out of her hair. The creature disintegrates into a pile of twigs and branches. With horror, the Doctor glances at Petra who is pointing to the screen in the corner and realises that the 'insect' was a diversion. There is a dark shape in Katy's room. The screen goes blank.

The Doctor runs from the room. Petra tells the volunteers that they need to get organized. She will follow the Doctor, the others need to secure the Grange and get the 4x4 vehicles ready. She makes her way to the nursery where the Doctor is opening the door. A small man is bent over Katy but he appears to be made of vegetable matter. Petra speaks, attracting the creature's attention. It runs at the door and the Doctor slams it. From the other side a hoarse voice says, "Come under the bridge with me." The Doctor is delighted by the creature until a thump inside alerts him. There is no sign of the creature when they enter the room and the only way out was through the window. As the Doctor leaps to a drainpipe and drops to the ground the floodlights come on, lighting up the grounds. Petra sees the creature break from the shadows and realises that she is not going to follow the Doctor.

The Doctor chases the creature across the lawn. To Petra it looks like it is carrying a blanket. Then she notices that Katy's bed is empty.

The troll outruns the Doctor who slows to a walk in the darkness. The wind is whipping up the topsoil and he thinks a dust storm is on the way. He almost trips over Katy in the road. She is grinning with delight she tells him that Karl came back for her. The dust rolls in. his only choice is to go on. A 4x4pulls up next to them and Sanders and Petra get out. The Doctor says he wants Petra to go with him to the bridge. Campbell, in the driver's seat, says that is playing into the Doctor's hands. The Doctor orders Sanders to carry Katy back to the Grange. The dust storm is closing in. The Doctor climbs into the car as the wind and dust howl around them.

They pull up near the river bed and Campbell gets parkas, goggles and scarves for Petra, the Doctor and himself. They slide down the bank and follow the riverbed towards the bridge. Petra points out a lumpy shape moving on the edge of visibility. They pause. Nothing can be heard above the gale. Something big and bulky emerges from the gloom and wraps itself round Campbell, dragging him away. The troll leaps onto the Doctor and roots from its hands try to wrap around his face. The Doctor recognises the troll's face as that of Karl Baring. He is hit by stones flung by the wind and falls. The troll leaps onto him and does not let go.

The Doctor fights to prise its fingers from his throat and only succeeds when Petra swipes it with a heavy branch. The Doctor rips off his coat and tries to wrap it round the troll but this causes the soil around him to swirl and form tentacles that reach out for them. The ground opens and he realises that he is going to be pulled underneath the soil. Petra takes off her coat too and wraps it around the troll. The troll goes limp and the tentacles dissolve.

Suddenly, out of the storm and darkness, a large helicopter land and armed soldiers leap out. Their officer, Lt. Chavez, says that the Grange said Petra needed help. The Doctor and Chavez bundle the troll into the hold and climb aboard. Petra sees Hamilton Bryant aboard and the minister orders the helicopter to take off. It swings around in the sky and launches high explosive and napalm missiles at the area around the bridge. The ground erupts into flame.

Later, in the common room, Chavez peels the coats off the troll. The Doctor asks to have the creature hooked up to the AMU. When Bryant asks why the Doctor tells the minister he is getting in the way and Petra tactfully escorts him out. As they fix the wires from the AMU to the troll Tim recognises that it is Karl. Vanessa says they should kill it but the Doctor adds that he doesn't think that would do any good.

The storm winds down as morning breaks. The daylight seems to discomfort the troll and it wakes and groans. The Doctor tries to engage it in conversation. There is no response to this apart from "Come under the bridge with me." The Doctor wonders if this is the troll speaking or some other intelligence that is talking through it. He tries several more questions including asking what it calls itself. Again there is no reply but when he asks what Karl Baring would say it was called it replies, "Sphereosis, that which feeds from below." It adds that it fell, long ago, and has been growing since, taking nutrients from the soil until it broke the surface and found oxygen and nitrogen. Then it grew quicker. After this the troll falls silent.

In the office, Bryant asks Petra what she is scared of. She realises that the relay to the outside world wasn't sabotaged; it was Bryant who cut the Grange off from the rest of the world. He agrees, pointing out that he did it to save her career. She says that the Grange is finished, that the troll is Karl. Bryant disagrees, saying that he is going to find Karl and take him back to the tropics.

The Doctor calls everyone back to the common room. He tells everyone that the troll is Karl. He was being used by some sort of plant life, but wrapping the coats around it cut it off from a complex system of roots. He adds that the Sphereosis has been growing for billions of years, gradually creeping to the surface and learning all the time. Now it is massive, though he refuses to speculate how large it is. This size, however, is working against it. Since the food disappeared it has grown hungry and has had to hunt for other food, hence the disappearance of Charlie and Campbell. Bryant argues that the Sphereosis is dead now but the Doctor thinks it is still alive and has learned to be a predator.

The troll revives briefly and drops to the carpet. Petra can clearly see Karl's face in the troll's as its skin falls apart. It whispers a final "under the bridge' and then crumbles into pieces.

On the roof the Doctor and Petra watch the smoke billow from near the Bridge. He thanks her for rescuing him from the troll. He says he is going to need her help but she thinks that she isn't brave enough.

Bryant calls a meeting and says he wants Petra and Katy on the helicopter and the Doctor placed under arrest. Surprisingly, the Doctor is delighted. Petra objects but Bryant ignores her and asks Tim to do an autopsy on the troll. The others are being confined to The Grange.

Chavez locks the Doctor in the cellar. There is no light. The earth in the cellar is moving. The Sphereosis is under the house. The Doctor climbs onto a table.

Katy is helped out of bed. Her hair is now pure white. Bryant wants to take her to the bridge to find her brother. Bryant hears that Sanders has left the house to walk to the coast and find a boat. Petra says this is suicide. Tim agrees, saying that when Sanders gets into trouble he is going to send up a flare to show them how far the Sphereosis stretches.

When she refuses to let the soldiers take Katy from her room Bryant orders his men to shoot Petra. She backs down, unable to stand up for her principles.

The Doctor hears the helicopter leaving and looks for a means of escape. He knows that the people on board are going to need his help very soon.

In the helicopter, Bryant tells Katy she is on her way to see Karl. Petra interrupts to say he is dead. Bryant denies this and Petra subsides into impotence. The helicopter lands on the edge of the burnt soil five hundred metres from the bridge. Katy steps out shakily among Bryant's team. The helicopter takes off. Far away a flare arcs into the sky.

Katy makes her way to the bridge, calling for Karl. The soldiers are spooked as she goes down the slope. Petra calls for them to stop her. Katy glides along the glassy soil to the bridge. Petra runs after her but Chavez shoots her in the back and sends her tumbling down the slope.

The Doctor leaps for the stairs as a tentacle erupts from the floor and smashes the table he was standing on. It feels around for him. Just as it is about to find him Vanessa opens the door and the Doctor leaps out. She slams the door closed and stands there, trembling. He grabs her hand and they run. The floorboards of the house are snapping throughout as the plant looks for food. They creep to the laboratory as quietly as they can. The walls are trembling in the corridor. In the laboratory they see Tim being sucked down beneath the floor. He is wrapped in vegetable limbs that are drinking him alive. It is too late to save him. The Doctor tells the plant that it has lived too long and he is going to stop it. More tentacles try to grab them. The Doctor realises that the plant has got part of Karl's genes within it. He appeals to this part of it to let him rescue Katy. The tentacles stop.

They reach the garage and drive off in a 4x4 as the house collapses behind them. Vanessa tells him she doesn't want to go to the bridge. The Doctor says that he has realised that the troll was trying to save Katy all along. It was taking her away from the bridge. He tells Vanessa to take the car to the coast and warn everybody to leave Britain alone and let the Sphereosis starve. She drives off, crying in shame.

Katy realise that Petra has been shot; this is the first thing she has known for a long time. She recognises where she is and runs towards her friend. She sees Petra is still alive. She wants to help. Petra is shivering. Katy takes her hand, aware of men gathering at the top of the bank. Petra tells Katy not to go under the bridge so Katy turns and runs.

Bryant is furious that Katy is disobeying him. He tells Chavez to catch her and get her under the bridge. Chavez is nervous and says they should go home. Chavez orders Corporal Stamper and Doctor Stevenson to go and catch her. They make their way after her. Bryant and Chavez follow. The five of them reach the bridge. Three soldiers stay at the top of the bank. Katy is defiant and angry. She tells Bryant that she knows her brother is dead. Bryant slaps her and says he won't be held responsible for the failure of The Grange. He denies that monster plants exist. Stamper and Stevenson march her to the bridge. Thin arms reach out and drag Stamper under the soil. Stevenson tries to escape but balls of twigs swarm out and cover him. The ground opens up and he is gone. Katy runs back to Bryant who orders Chavez to get the helicopter back. Chavez simply runs away but the creatures drag him down, too. He fires at them with his pistol.

Bryant sees that Katy is not being touched and grabs hold of the girl. Chavez shoots himself as he is dragged under. Bryant begs Katy to save him. A 4x4 squeals to a halt near them and the Doctor lets them in. a gigantic tentacle rises up from the ground. The Doctor asks where Petra is and Katy tells him that Chavez shot her. The tentacle has grown as large as a block of flats. The Doctor thinks it has given up on catching them. It continues to rise into the sky. The Doctor stops the car and gets out. He says that Karl is forcing the Sphereosis to waste all its remaining energy in order to protect his sister.

For a few moments there is silence. The tentacle stops growing and shudders. A fountain of earth signals a head rising from the soil. The troll wriggles out of its hole. Bryant leaps into the car and drives off. More trolls begin to come from the soil. The Doctor says that this is the plant trying to get round Karl's influence. The only way to keep Karl fighting is to put Katy in danger. He says they need to go under the bridge.

Trolls leap onto the bonnet of Bryant's swerving car. He hits the bank of the river and airbags explode around him. More troll emerge and study him through the window. The vehicle lurches downwards and feelers come in through gaps in the car's structure until it is full of fibrous roots.

Under the bridge all is silent. The Doctor and Katy look around them. The tentacle is sliding quickly towards them. Katy tells Karl that she loves him. A troll arrives and tells her that there is no Karl but she repeats that she loves Karl. It staggers away, calling Katy's name. The Doctor grabs the troll and pushes it to Katy. She sees Karl's face as the troll disintegrates, as does the approaching tentacle.

The country is silent. Only dust moves in the wind. The tentacle is now a huge mound of soil. Katy and the Doctor crawl out of the soil. He tells her that Karl and Katy killed the Sphereosis. They see that Petra is still alive. The Doctor says she needs medical attention to which Katy replies that she was once a medical student. Just then the helicopter heaves into view and lands beside them. It will have the medical supplies that Katy needs for Petra.