Archaeology student Angus Flint unearths a prehistoric skull while excavating the grounds of Mercy Farm, a guesthouse in rural Derbyshire. He explains to Mary Trent (who runs the guesthouse with her sister Eve) that, although it looks like a dinosaur, it cannot possibly be as it was found at the same level as some Roman coins and the two would not have mixed. The excavation also reveals that a convent existed at the same site at the time of William the Conqueror. Mary invites him to accompany herself and Eve to a party that evening.

At the party, an annual event held for the locals at D'Ampton Hall, Angus learns from James D'Ampton, the current Lord of the Manor, the legend of how brave John D'Ampton slew the D'Ampton Worm and saved the villagers - an event acted out through drama and song. James explains that the word 'worm' should not be taken too literally, as it is probably a derivation of the Anglo-Saxon 'wyrm' (also German 'wurm', Gothic 'waurms') all meaning 'dragon' or 'snake', but that it is worth remembering that the lowly earthworm used to be of greater significance.

Eve stays on to dance with James while Mary and Angus walk home, taking a shortcut through the wooded grove. We learn that both Mary's parent disappeared in this area nearly a year before when returning home from the pub, and that no trace of them has ever been found. We also discover that Mary is now single, her boyfriend having been killed in a motorbike accident. Just as she and Angus are on the verge of intimacy a strange car with dipped headlights crawls past them, heading towards Temple House, the home of Lady Sylvia Marsh.

The local policeman, Ernie, is waiting for them at the farmhouse, as Joe Trent's pocket watch has been discovered at Stonerich Cavern. A search party is being arranged for the following day to see if any further clues can be found. As he is leaving Mary informs him of the mysterious car, so he heads for Temple House to investigate.

After failing to gain assistance (by walkie-talkie) from his deputy, he is suddenly bitten by a snake, at which point Lady Sylvia Marsh appears asking if he requires any assistance. Inside her house she sucks the venom from his ankle and inquires as to why he was in the grounds. He then tells her of Mary noticing the car and that Joe Trent's watch has been found. 'And was he on the end of the chain?' is her unconcerned reply.

The following day Sylvia drives to Mercy Farm and snoops around the house. She first checks the watch, then goes upstairs and steals the skull from Angus' room. On the way out she notices a crucifix on the wall. Her eyes turn yellow, her teeth to fangs, and she spits green venom over the cross and wall before driving away, taking the skull with her.

James drives Eve home from the party, and they find the watch note from Mary and Angus saying that they've joined the search party at Stonerich Cavern. Eve says she'll have a lie down until they get back, and James goes to take some pictures of the excavation before returning to D'Ampton Hall. On the way up to her room Eve notices the venom and touches it. She immediately falls unconscious and experiences a surreal dream where nuns (one being herself) are ravaged and murdered by Roman soldiers while worshipping Jesus on the cross, who becomes entwined by a large white serpent. Throughout all of this a vision of Sylvia, painted blue and showing her pointed teeth, laughs at the carnage. James, hearing her cry, returns and asks her to explain what happened. She can't remember until she removes Joe's watch from her pocket and sees that the hands have turned into snakes. She explains the image of the crucifixion/serpent and James insists she looks at the excavation - where Angus has uncovered a mosaic depicting a snake wrapped around a cross. While they are searching for the skull Angus and Mary return from their unsuccessful trip.

Sylvia, having deposited the skull, goes snake-watching in her car (in the rain) and picks up an innocent hitch-hiking scout named Kevin. She insists he must come back to her house to dry off. They play snakes and ladders, then his mouth-organ music causes her to dance strangely. She grabs it from him and insists he takes a bath before 'the experience of a lifetime', passionately kissing him to seal the bargain. In the bath she scrubs him down and insists he should stand up to enable her to wash him properly. When he does, her fangs promptly reappear and she bites his genitalia, injecting venom that paralyses his nervous system. She then informs him that he is to be fed, alive, to the god Dionin, and that this is a great honour reserved purely for virgins. While elaborating about the Dionin cult, the doorbell sounds and she is forced to drown him as a precaution.

The sounder of the doorbell has been James who, after seeing Ernie's ankle, is making an offer to have her grass cut to scotch the snake. Previously unacquainted, Sylvia thanks him and invites him in for a brandy. They make amiable conversation, she speaking of her love of playing solitary snakes and ladders (as the board is still present) until the subject of the Trent's disappearance arises and Sylvia quips 'To lose one parent may be regarded as a misfortune, to lose both looks like carelessness', at which she laughs, though James finds this remark in bad taste. She justifies herself by claiming that humour is her defense mechanism and she has been having a difficult time - she explains that she is terrified of snakes so would never have been able to attend his D'Ampton Worm party at the Hall, and that Ernie's snake bite greatly upset her. He asks why she should then play snakes and ladders and she says it is a compulsion - she was in a coma for ten days due to a snakebite when a child and has had a fear of (and morbid fascination with) them ever since. He recommends facing this fear, so she throws the snakes and ladders board into the fire. As he is about to leave, they passionately kiss.

James, in his bedroom, agrees with Mary (by phone) to meet up the following day, before he falls asleep. As he does so, he imagines that the worm has disappeared from the historical painting of the D'Ampton Worm, himself appearing in its place. He enters the cave in the painting and is transported into an aircraft where the stewardess' are played by Mary, Eve and Sylvia - the only other passengers being Dorothy and Joe Trent. He starts to draw around the outlines of a crossword puzzle and finds that this creates the image of the serpent wrapped around a crucifix. Sylvia then attempts to make him drink some green liquid, though Eve wrenches it from her. They fight, but just as Sylvia seems to be winning the fight ends and Joe Trent leaves the aircraft. James is now faced with two exit options (both signaled by a stewardess' leg) of either heading in the direction of the green liquid, or towards a ring depicting the crucifixion. He heads towards the ring (in the same direction as Joe) and emerges in Stonerich Cavern, where he finds Joe's watch. At this point Peters wakes him up with his breakfast tray. He scolds him for breaking his dream, but then is surprised to notice, from the picture in the local paper, that the cave in his painting is actually Stonerich Cavern. Peters informs him that this was the traditional home of the D'Ampton Worm of legend.

James, Mary, Eve and Angus head off for a further search at Stonerich Cavern. James is convinced that there must be a link between the watch being found in the Cavern and the legend of the D'Ampton worm. The others think he is mad, reminding him that John D'Ampton made halves of the worm many centuries before - 'Yes, but what happens when you cut a worm in two?' is his undeterred reply. James and Angus look at a primitive cavepainting, depicting hermaphrodites, a similar sexual state to the earthworm. James insists on seeing the place that the watch was found, as he is convinced that it could only have reached that place by being swallowed by the snake then passed undigested into the caves. Eve heads off alone to prepare the evening meal for the guesthouse residents.

While walking through the grove Eve meets Sylvia, who claims to have been caught in a tree while helping a kitten. Eve lends her a hand down, and once Sylvia is back on the ground Eve appears to be in a sort of trance, and blindly follows Sylvia's invitation back to her house.

At Temple House, Sylvia orders Eve to 'disrobe' before beginning a talk on reincarnation. She reveals that she is immortal and has seen Eve's soul many centuries before, when Eve was a Roman nun. She mocks the Christian faith - 'captive virgins in the hands of an impotent god', and talks of the revenge she took on Eve's order when they attempted to build a convent on the side of her temple, and also how Emperor Marcus Carousius was her lover. Stopping Eve before she is naked, Sylvia talks of how only virgins are really acceptable sacrifices, then tests her virginity with a large horn dildo. Passing the test, she informs Eve that she will have the honour of being sacrificed alive. Sylvia then commands Eve to ring Mary and tell her that she's gone to London for a few days. Eve gives the concerned Mary a somewhat stilted speech, but the sight of her crucifix ring releases her enough from Sylvia's spell to shout 'Dionin', at which point Sylvia spits venom across the room, making Eve fall into a fit and causing another hallucination featuring the Skull and skewered nuns.

Back at Mercy Farm, Mary returns the phone receiver not really understanding the conversation she's just had, and that Eve shouted 'Dionin', a phrase she has never before used. Mary tries to ring the railway station to try and talk to her while Angus and James discuss how Dionin was a pagan snake god and that Eve had been muttering something about him after her previous hallucination. Mary is then convinced, since none of Eve's belongings are missing, that Eve must be being held somewhere. James expresses his concerns that, since Eve has never wandered off or mentioned Dionin before and that a skull found on the site of a pagan mosaic was stolen, her disappearance could have something to do with a pagan god. Mary, when Eve's hallucination is mentioned, notes that Eve is quite religious, so James then decides that the whole situation could be a battle between Christianity and Paganism, and that Eve may well be being held for human sacrifice. James says there must be a connection between Temple House and the pagan religion, as there was a connection between the D'Ampton Worm and Stonerich Cavern. An incredulous Mary taunts James that he'll be saying there's a connection with Ernie snakebite, so he then mentions snakes and ladders. He is now convinced that the connecting factor between all these things is Lady Sylvia Marsh.

That night James has his butler Peters place loudspeakers on the roof of D'Ampton Hall before playing a Turkish snakecharming tune at full volume. Sylvia, at Temple House, emerges like snake from a basket and is charmed from the house, leaving the coast clear for Angus and Mary to investigate. They find a bust of Marcus Carousius, the Rebel Emperor, in Sylvia's hallway. They enter a darkened room where a lone figure watches a snake charmer on television. Mary sees it is her long-lost mother, but before she can make conversation her mother bites her in the neck. Angus chases her off with a fire brand and sucks the poison from Mary's throat, but not before she has had a hallucination where blue-painted hermaphrodites rip her apart with sharpened strap-on dildos. Angus then rings James to tell him the plan is working as Sylvia is on her way. James then tells Peters to leave the door open and lock up the chambermaids, but moments later there is a powercut and the sound of Peters screaming. Before James can discover what has happened he hears a high-pitched battle cry and, seizing Jon D'Ampton's sword from the mantelpiece, swings it at Dorothy and slices her in two parts, which continue to move. James runs out to the stairwell to find Peters dead in the hallway just as the power returns. Angus calls and tells him how Mary was bitten but is unharmed, then asks why there is no music playing. When James looks the record is missing from the player. 'Thanks for the warning - I think we probably have another reptile here on the premises' he says, before looking down and finding half of Dorothy grabbing his ankle. Angus then tells Mary, now safely home in bed, that her mother is at rest.

The next day Mary bandages the puncture marks on her neck before meeting Ernie, who says she must come down to the station and identify her mother's body. Ernie talks of how he found two corpses at D'Ampton Hall, and that James is helping them with their inquiries. Mary then says she wants to place a missing persons report for Eve. Ernie seems concerned, then adds that they are picking up Sylvia at Temple House due to James' allegations. They draw up outside the house and Ernie, leaning on Mary's door, reveals a bandage on his wrist. Mary automatically realizes that he must have been bitten and whacks him with the car door before trying to run out of the grounds. She gets trapped by the gates then blocks herself into a corner. Ernie, now with snake features, gives chase but is charmed by the sound of Angus' bagpipes before he can manage to bite her. Angus continues to play, luring him to a sundial at the front of the house. Angus eventually has to stop for breath, at which point Ernie tries to attack him, but he pushes him back with the bagpipes and Ernie ends up with his face skewered on the sundial.

Angus, still playing, enters the house, injects himself with snake venom antidote and releases a mongoose (traditional enemy of the snake) then follows after it, only to be hit in the face by the skinned animal then bitten in the knee by a blue-painted Sylvia. She, laughing, removes her ear-plugs and drags him unconscious into the basement, where a scantily clad Eve is suspended over a well and Mary is tied and gagged. Through the well we hear the sound of James and some men from the village who are about to try to smoke out the worm. Sylvia walks around the well preaching devotion to Dionin, then places a wreath on Eve's head before walking up the well steps, towards her, wearing a strap-on horn dildo. Before she can commit her ceremonial rape on Eve, the worm, stirred by James' smoke, begins to emerge in the well. Sylvia starts to cut Eve's ropes so she can be fed to the worm. Suddenly, Angus wakes up and pushes Sylvia into the well. She grabs onto Eve's feet, so Angus saws through her wrist until she falls into the mouth of the worm. Angus removes Eve from the well and unties Mary before throwing a grenade into the worm's mouth.

Outside, he injects Eve with the antidote as a precaution and sends them to hospital. Back at Mercy Farm James turns up and explains to Angus that he smoked the caves as he though there was an underground link to Temple House. The phone rings - a nurse from the hospital who produced the antidote for Angus tells him that they have provided him with the wrong serum, one that would be no use at all against snake bites. Angus hangs up and feels his glands and teeth. He joins James in the car who suggests that, since they both must be hungry, they stop for a bite. 'Why not?' is the reply, as James pulls back the gear stick, catches Angus' kilt and reveals the bite above his knee....