Nigel Carson and his friends Ben Seddon and Duncan Goode are tunnelling for two million pounds in gold. Carson is the leader of the trio, Seddon is the brains and Goode provides the muscle. They are discussing what they will spend their money on but Carson is more interested in the power that their treasure will yield. He has a stone that he clutches, a stone that is covered in fine tendrils and seems to converse with him telepathically.

Martha and the Doctor are in the woods nearby, glad to be in twenty-first century England again and hoping to find somewhere that will sell them a cream tea. They come to a road with a signpost telling them that Creighton Mere is a mile away. An old tramp on a stile in the hedgerow warns them to avoid the village but when Martha asks why he can only tell her that he has a feeling about the place. Soon after, a Land Rover stops to give them a lift, driven by an old woman in a camouflage jacket. She drives quickly because she is 83 and has no time to lose. Her name is Angela Hook. She tells them that the old tramp they met was Old Barney. They pass a Georgian manor which Angela honks her horn at, ‘out of habit’. She says Henry Gaskin lives there and it is her duty to be as much of a nuisance to him as possible.

She parks by the village green and they see a small village with a church and a pub, The Drinking Hole, and a well. The well is two metres wide with a waist-high wall around it. She takes them to look at it, saying her current project is to renovate it as a tourist feature. They are introduced to another old woman, Sadie Brown, another part of the renovation team. The four of them make their way to the pub where the Doctor says he wants a dandelion and burdock, but not before he has cast a perplexed glance at the well.

In the pub they discuss the legends that surround the well, including one that concerns a highwayman’s treasure lost down the well in the eighteenth century. Angela Hook thinks the treasure was probably found, and spent, two hundred years earlier by the Gaskin family.

As the evening sun sinks the trio of Carson, Goode and Seddon make their way past the well to the pub. They meet Old Barney who tells them they are not wanted but Carson replies with some cruel jibes. As they enter the pub the barmaid, Lucy, is telling the Doctor that Carson’s reason for being in the village is research for a tourist guide, but everyone thinks they are looking for the treasure. The Doctor asks Nigel Carson if they have nearly found it. When Carson denies any knowledge of the treasure the Doctor says that as there is a metal grille over the well they must be tunnelling in from the side. The three young men head to the bar and the Doctor, on finding out that the repository of local legend is Old Barney Hackett, says he wants to have another chat with the old man.

Later, still in the pub, Seddon shows a schematic of the well shaft and their tunnel to Carson. It illustrates how close they have got to their target. Carson’s pleasure is tempered by the presence of the Doctor and by Martha’s friendly conversation with Duncan Goode at the bar. Carson suspects that Goode has told Martha or the Doctor about the tunnel, which is why he seemed so well informed about their doings earlier. Carson speaks of cutting Duncan out of the treasure, but only after he has dug the tunnel for them.

Martha leaves the pub to find the Doctor looking at the well. He tells her that it doesn't feel right to him, and reminds her of many stories where wells were guarded by spirits or monsters. They are interrupted by Barney Hackett who tells them that this well isn’t safe and that Tommy fell down it and died. It is soon revealed that Tommy was a cat. Despite being dead for six months the cat still calls to Barney, allegedly. In the pub Angela and Sadie are looking over plans for a windlass they are having installed on the well the next day, though they expect Henry Gaskin to interfere. Carson has retired to his room upstairs in the inn. He is looking from his window at the Doctor and Martha talking to Hackett. The stone in his hand says that the Doctor is a threat and it will prevent him from interfering in their plans. Nigel knows that this means the Doctor is about to die.

At the well Hackett is recounting some of the local myths. One concerns a highwayman who stole gold and jewels from the Duke of York and was pursued across the country until his horse went lame near the village. He tossed the treasure down the well and climbed in after them, hanging on by his fingertips. The militia was alerted to his presence by a barking dog and the captain, guessing what had happened, prised his hands from the wall. The highwayman drowned but had apparently sworn his vengeance. When the militia captain returned, ten years later, with his new bride (possibly with a plan to retrieve the lost treasure) he was murdered in his bed.

While the Doctor doesn’t necessarily believe any of this he has further questions but the old man suddenly has some form of fit. A green light leaps from his mouth and he starts to transform into a bony, spiny creature covered with a crackling green light. Then he turns to ashes and is blown away on the evening wind. The Doctor surmises that his body couldn’t cope with some form of molecular transformation and collapsed. The green glow was some form of telekinetic force field. Martha wants to tell the police but the Doctor asks her who would believe such a story. They return to the TARDIS as the sound of a cat mewing echoes in the well.

The next morning sees the Doctor and Martha back at the well. Sadie and Angela are there too, waiting for the delivery of the windlass. The Doctor offers his services to go down the well and see if there is any water at the bottom. He uses his psychic paper to overcome their doubts and convince them that he is from the Council’s Heritage Department.

In the tunnel Duncan Goode and Ben Seddon are digging, close to their goal. Nigel Carson is nervous, partly due to the death of Barney Hackett but more because the Doctor and Martha had witnessed it and then left without apparently reacting in horror. The stone has told him that its plan to use a transformed Barney to kill the Doctor had failed because of the frailty of the human form, but next time would be different. The digging is halted by the discovery of a human skeleton in the soil. Carson surmises that it is the remains of the legendary highwayman and orders the others to carry on digging.

At the top of the well there is a commotion when Henry Gaskin arrives, saying that it his duty of care to the village to prevent them damaging the well. The Doctor tries to show him his ‘papers’ but Gaskin says that he would already know of them if they existed and storms off. Sadie says she doubts Gaskin’s claim that the well is a protected building and feels that his interference is due to a feud between Gaskin and Angela, concerning the death of Angela’s husband when mountaineering with Gaskin nearly twenty years before.

The Doctor uses his sonic screwdriver to remove the grille over the well by the time the windlass is delivered. The young men who bring it are inspire to fix it in place by the sight of Martha, but all their impressive labour is in vain when she ostentatiously links arms with the Doctor. After the men depart the quartet by the well lower a bucket into the depths. After lowering it some way the bucket seems to get snagged on something, then is pulled down fiercely. The Doctor uses his screwdriver to snap the rope which is pulled up. The Doctor announces his intention to go down and investigate.

He is prevented by the return of Gaskin, telling them he has checked to see if any papers have been issued by the Council, but there were none. He is convinced that they have been seduced by tales of treasure and monsters. Before he can finish he is firmly put in his place by Angela and storms off, directing one last warning to the Doctor. The Time Lord is intrigued by the mention of monsters, something nobody has brought up before.

Angela dashes home to get some of her ex-husband’s climbing gear. The Doctor is put into a harness and given a powerful torch prior to his descent. He tells Martha that if anything goes wrong she should go to Henry Gaskin for help, which mystifies her.

In the tunnel Nigel Carson is feeling steadily more unwell and his friends tell him to go back to the fresh air outside. As he clutches the stone in the palm of his hand he finds that the hairs on it have risen and are penetrating his flesh. The consciousness within it is telling him that the time of ‘the rising’ is near and that it will soon be time to feed.

Sadie has given the Doctor a walkie-talkie so that Martha can keep in touch with him. At first this is fine but the lower he goes, and the more he sees a curious, pale fibrous growth on the walls, the weaker the signal is until he fades away completely. At one point the growth has become a fibrous mass blocking the well. As he fights his way through it he sees the corpse of a dead cat, presumably Tommy, and to his alarm it opens its eyes and mews at him. At the top of the well Martha hears an echo of the mew and is reminded of Barney Hackett’s demise. She demands that they winch the Doctor back up but the windlass is frozen. Down below the growth has started to attach itself to the Doctor’s feet and pull his legs. In an attempt to break free the torch falls from his fingers leaving him in darkness. When the women at the top eventually manage to free the windlass and haul the rope back up the harness is empty and the Doctor has gone.

Martha feels it is time to tell the two old women about Barney Hackett’s death. They are skeptical but eventually believe her. They decide to call the emergency services but Martha is set on going to the Gaskin residence. Meanwhile, the Doctor tries to find out what the growth is that is steadily tugging him down the shaft, but his sonic screwdriver can only say it is neither animal nor vegetable. In addition, the device seems to be making the growth tighten its grip on him. In the darkness, however, he notices that there is a dim glow from the growth, a kind of bioluminescence.

When Martha and Angela arrive at the Manor Martha is struck by how shabby the place is looking. Gaskin is not pleased to see them but when she tells him the Doctor has fallen down the well he invites them in. He is surprised that the Doctor told her to go to Gaskin for help, particularly because this was predicated by his use of the word ‘monster’. He says he has no intention of going down the well to rescue the Doctor, causing Angela to make some caustic remarks and storm out. Gaskin explains to Martha that the bad blood between Angela and him dates to the death of Roger Hook. He says that both men were in the parachute regiment together and were friends for life. When Roger found he had a fatal heart condition he declared he did not wish to die in his bed and planned one last climb in the Alps. On the way down the Jungfrau Roger had a heart attack. Gaskin found that Roger’s medication had been left in the chalet, probably deliberately, and there was nothing he could do for his friend. Angela, on the other hand, has always believed that it was Gaskin’s idea to go on the trip and his fault that her husband died. As to the well, Gaskin merely suggests that the police are Martha’s best hope.

At last the Doctor is deposited at the bottom of the shaft. He can hear an irregular thumping as if from diseased heart, and see pale lights like eyes in the darkness. He recognizes that these eyes hold no compassion, only malevolence and hatred. He also knows what it is that he is facing.

Martha finds Angela outside by the Land-Rover and tells her that Gaskin refuses to help. Angela suggests that they go round the back of the house to find some of Gaskin’s old climbing gear. At the side of the house, on a terrace, they find Nigel Carson, collapsed and unconscious. Suddenly they are disturbed by Gaskin and his border collie, Jess. As the trio help Carson into the conservatory Martha picks up a small, translucent stone which is lying nearby. Jess is terrified of the stone but Carson snaps back to life and grabs it back. He hugs it to his chest as if it is comforting to him. When Martha questions him about it he wraps it in a handkerchief and puts it back into his pocket. Gaskin returns with a first-aid kit and tells them that Carson is his guest. Angela realizes that the tunnel Carson has been digging is the old Gaskin Tunnel.

Gaskin confirms that Carson has been renovating a tunnel started in 1902 by Gaskin’s great-grandfather, Rupert. It was an attempt to find the treasure but was abandoned when Rupert died of influenza. The need for money which inspired the excavation was simultaneously overcome when a distant relative died and left a fortune to the Gaskins. However, Gaskin Manor was used as a convalescent home in the Second World War and Carson’s grandfather was stationed there. He found the documents relating to the tunnel and passed the details on to Nigel Carson before his death. Martha decides that the best way to get to the Doctor is down the tunnel and sets off. Gaskin refuses to follow, leaving Angela to contemplate a journey of nearly half a mile in the underground darkness to help Martha.

At the same time, Goode and Seddon have reached the brickwork of the well. Their joy is short-lived. Ben Seddon has just told Duncan that Carson was planning to cut him out of the treasure when the skeleton they found grabs Duncan by the throat. As Ben wrestles with the skeleton to prevent it killing his friend a green spark jumps from the skeleton’s mouth towards Duncan. The skeleton disintegrates into dust and Duncan begins to transform: his teeth become fangs, his skin is webbed and bulging grey veins are evident beneath the surface. Martha arrives at the moment that the monster kills Ben Seddon. Green energy crackles from his mouth, encapsulating the victim and turning him to a blackened corpse that crumbles to dust. Duncan sprouts angular spikes from beneath his skin and turns to attack Martha. She ducks away from him but finds herself trapped between him and the wall of the well.

Help is at hand when the Doctor pushes through the crumbling wall and helps Martha evade the creature that was Duncan. They run up the tunnel and encounter Angela. Grab her and continue. The old woman slows them down and the creature begins to close the gap. Martha tries to find some trace of Duncan within the monster by reminding him of their conversation in the pub the previous evening but it is to no avail. The Doctor provides the solution by pointing his sonic screwdriver at a support beam in the ceiling and bringing the roof down on the creature. Unfortunately the roof continues to fall and collapses on the three fugitives.

In the manor house Gaskin is growing more and more upset. Initially it is Carson’s unconsciousness which bothers him, then his dog’s fear of the stone, then a growing sense that the stone is watching him. Finally he is troubled by the thought of Angela disappearing into the mouth of Gaskin Tunnel. This last anxiety is what drives him to follow her, despite his fear of dark places. He encounters the roof fall but has no time to react before the Doctor, Martha and Angela emerge from the loose soil under a protecting beam.

Back in the house the Doctor tells the others that the monster in the well is a Vurosis, a proto-molecular parasite from the Actron Pleides star system. It would have arrived as a seed then mutated the life forms it encountered on its way to world domination. He tells them that he is puzzled by the lack of intelligence in the creature he met underground, until the stone that Carson was carrying is mentioned. It is then that he realizes the Vurosis has been separated from its brain. He revives Carson who is bereft now that the brain has stopped speaking to him. The Doctor surmises that the stone left Carson and took control of Duncan because of his physical superiority. It transformed him using the lessons it learned from the failed attempt on Barney Hackett and then killed Ben Seddon to show off its strength. Now the creature in the well is going to use Duncan to reunite the body with the brain.

Sadie interrupts their conversation with a welcome delivery of food in a hamper and the Doctor devours a jar of her marmalade using only his fingers. Nigel confirms that his grandfather found the stone in the Gaskin tunnel when he was investigating the plans he found during the war. The Doctor suggests that the stone has been altering the way Nigel’s mind works, imperceptibly, as it seeks to fulfil its plans. Nigel agrees that by the end he no longer thought of the treasure as gold or jewels but as power of some kind. The Doctor decides to communicate with the stone by using his screwdriver but this makes it fire green light at him, apparently killing him. Martha, however, knows something of his physiology and they leave her to look after him in the kitchen.

Angela and Gaskin talk through her husband’s death and come to a reconciliation which prompts him to fetch his shotgun and finish the situation by blasting the stone. Just as he is about to pull the trigger the Doctor makes a spontaneous recovery and stops him. He says the best plan is to get the thing off the planet and drop it into the middle of an asteroid field. Just then, the dog barks and the creature that was Duncan smashes through the back door. Everyone flees up the stairs and into a bedroom. As the creature breaks that door down too, the Doctor heads to the en suite bathroom. The creature takes Sadie hostage but the Doctor flushes the stone away down the toilet, saying that if the monster in the well is ever reunited with its brain the planet will be lost.

This causes the creature to abandon Sadie and burrow down through the house after the stone, smashing through walls and floors towards the sewers. Sadie begins to emit green light and go into fatal molecular transformation, but the Doctor stabilizes her with his screwdriver. Nigel Carson brings the bad news that the creature has made it to the sewer, found the stone and is on its way to the well.

Carson runs off and steals Gaskin’s Daimler, driving off in pursuit of the stone. Martha drives the Land-Rover with the Doctor in the passenger seat and Angela and Gaskin in the back. Everyone arrives at the well at the same time. Nigel pleads with the creature and the stone, needing to restore his psychic link. The Doctor stops Gaskin using his shotgun, pointing out that somewhere in the creature is Duncan Goode, an innocent in all this. In the interim the creature drops the stone down the well and collapses, then returns to the shape of Duncan. He has no memory of anything that happened since the skeleton attacked him.

All this commotion has brought a lot of villagers out of the pub. They are alarmed by earth tremors and the appearance of hundreds of worms squeezing out of the soil around their feet, trying to escape from the evil underground. A green shaft of light is cast from the well and then the Vurosis begins to grope its way out. It is a huge creature, tentacled and spiny, oozing mucus. The Doctor is transfixed, at a loss for anything to do. He doesn’t even interfere when Gaskin blasts it with his shotgun. The Vurosis retaliates by sending thin white weeds up from the soil to capture and submerge the onlookers. The Doctor and Martha run from one to another, sometimes pulling them free, but just as often arriving too late. In the confusion Nigel crawls to the monster and pleads to be recognized. It tells him that he was useful for a while before murdering him, his body crumbling to dust.

The Doctor demands to know why it did this. The Vurosis is intrigued that the Doctor is alien to the planet and by the fact that he knows its origin. It does not kill him, however, but begins to grow a thick, thorny dome around the village green. The Doctor and Martha are perplexed by the fact that they are still alive until it occurs to the Doctor that the monster is short of energy and needs to build a defence with the reserves that it has and cannot waste more on killing anyone else. Yet.

He tries to provoke an attack on himself which eventually causes the Vurosis to transform him, but the Doctor uses his mental strength to hold back the effects. After a while the monster gains the upper hand and the Doctor’s body begins to change, so he implores Martha to take his hand and join with him in the fight. Angela, Gaskin and Duncan have found themselves outside the thorn barrier. Knowing that the Doctor and Martha are probably dead, but possibly in need of help, they drive the Land-Rover through the thorn barrier and towards the well. Seeing what Martha and the Doctor are doing they join hands with them and together they turn its energy back on itself. The Vurosis blackens, splits and crumbles away to dust.

Across the green the people that were trapped in the alien weeds are revealed again. Martha goes from one to another, helping to revive them. The Doctor and Gaskin find the remains of the Vurosis brain. The Doctor crumbles it to ash between his fingers. As they survey the wreckage of the green and the well Gaskin comments on the mess that this event has created. When the Doctor talks of how easily things can be repaired Gaskin explains he was thinking of the people who died. The Doctor sighs and tells him it is better to think about the people who survived. Angela joins them and says Sadie has called her, saying she has made a full recovery, but Gaskin Manor looks as if it has been bombed. As they examine the damage that the well has suffered they realize that the grass is littered with gold coins. Angela says that it is not the treasure but the Creighton Mere Well and Gaskin Manor Restoration Fund.