Three friends leave their high school at the end of Friday’s lessons. They are Rick Pirelli, his under 13 football quarterback friend Scott and the rather studious friend Thad. They are excited because this is the day before Halloween. They have plans for trick or treating, a trip to the Blackwood Falls Carnival (including the ceremonial burning of the Pumpkin Man) and a night of scary movies. Everyone in Blackwood Falls seems to love Halloween and decorates their house lavishly.
Rick lives in an old clapboard colonial house with a picket fence. His mother is still young and attractive enough to cause Scott to blush and she offers them a ride home later, when they have chosen their Halloween costumes.
As the boys are discussing this they notice a curious green light coming from the tree at the back. This large and unusual tree is the black wood after which the town was named. They decide to investigate, a process which ends with them digging for ‘treasure’ under the tree. They are amazed to unearth a large and ancient book. It is written in a coded or ancient language, yet Scott seems able to read it after slipping into a trance. When Rick, somewhat spooked, snatches the book away it glows green briefly.
Inside his bedroom Rick deposits the book beneath his bed. After a brief fright when his elder brother Chris leaps on him, dressed as a monster, Rick takes his friends downtown for their costumes. Meanwhile, Etta Helligan, their elderly neighbor (a woman with a reputation for prescience) senses a disaster looming for the whole town.
The Doctor catapults from the TARDIS brandishing his sonic screwdriver, pursued by Martha, and races around the town searching for the source of an energy splurge. He says the source of the energy is very old. He tells her they are in New England and Martha notes the Halloween displays but thinks that the greenish mist pervading the town is taking things too far. He tells her that the mist is not from Earth but is not toxic.
They arrive at the Pirelli residence and make their way to the back garden. At the base of the tree they discover a recently dug hole. Investigating it causes the tree to attack them with spiky growths that the Doctor wards off with a flamethrower concocted from a candle and his sonic screwdriver. They conclude that the energy splurge was triggered when whatever was in the hole was removed from the ground.
When Mr. Pirelli disturbs them the Doctor’s psychic paper convinces him that they are working for the Environmental Health and Safety organization. He tells them that his son dug the hole and can be found at Tozier’s costume emporium.
Rick has a werewolf costume in his bag, Scott has chosen a Frankenstein outfit and Thad has a mummy/ghoul costume. Crossing the square to Harry Ho’s ice-cream parlour they encounter Dr. Clayton. Clayton has become the town drunk since his wife died. While his friends mock the old man Rick feels a sense of guilt. In Ho’s they are accosted by the Doctor (devouring a banana split) who demands to know what they dug up out of the garden. He tells them that they are in grave danger and that they caused the green mist that is filling the streets. They eventually confess to having unearthed a very old book.
Meanwhile, the book under Rick’s bed glows bright green and then vanishes.
Earl Clayton’s drunken walk home takes him past the graveyard. A noise within causes him to investigate but he is soon enveloped by the mist and begins to feel greatly afraid. A terrifying figure emerges from the ground, stick thin and with immensely long razor-tipped fingers. Its head is huge and pale and full of jagged teeth. Fleeing from this monster Earl runs into another. It makes a slashing motion in the air and he feels something awful happening to his face.
Chris Pirelli wakes from a nightmare of being buried alive. He goes for a drink of water and sees a green glow by the old tree in the garden. Fighting his fear he goes to investigate and sees another monster, similar to Earl Clayton’s assailants, disappearing into the ground.
In a nearby motel the Doctor is brooding over the disappearance of the book from under Rick’s bed when he notices Earl Clayton outside, collapsing in the street. Racing to the old man’s help the Doctor discovers that Clayton no longer has a mouth. The motel’s owner, Eloise Walsh, takes him in while the Doctor and Martha retrace his steps.
Martha’s deepening sense of dread is explained by the Doctor as an effect of the mist. Ultimately they encounter Etta Helligan in the graveyard. She tells them she was drawn to the place by a sense of foreboding. Suddenly, six monsters are created from piles of leaves and pursue the trio, attacking with razor-sharp leaves. The Doctor uses the salt from a blue bag in a packet of crisps to turn the creatures back to leaf piles. Etta describes it as a natural occult defence, impressing the Doctor.
The alien monsters have gathered in an underground chamber. At the centre is the ancient book, flickering with green light. Its light flows down to the tendrils and nodules that cover the walls and floor. Then the leader takes a ball of light from the book and sends it drifting away.
Jim Tozier sits alone in his costume emporium. He is mulling over the recent death of his partner Glenn as well as his decision to sell the business and move to Vancouver. He is troubled by the mist outside and a growing sense that the costumes around him are starting to move on their own.
In Etta’s house the Doctor is learning some local history. The old Blackwood tree fell from the sky, according to Niantic legend, rather than grew from the ground. When she says her family and ancestors who grew up in her house, above the roots of the tree, are renowned for their psychic gifts the Doctor grows concerned. He starts to use his sonic screwdriver to search for clues when Etta’s cats (of which there are many) become possessed (or ‘holistically subjugated’ as the Doctor prefers it) and attack the three people in the house.
They flee into the cellar where the Doctor is distracted by a door in the floor leading to a fruit store. Then they climb out of a window and head back to the hotel for a cup of tea.
The following morning sees Chris Pirelli arguing with his parents. Not because he does not want to help his father set up the carnival as they suspect but because he is still spooked by his sighting of the monster in the garden the night before. On his way into town Chris encounters the Doctor and is lured into a conversation. He eventually describes the monster which the Doctor identifies as Hervoken.
Martha has spent an uncomfortable night in the bath after giving up her bed to Etta Helligan. She had hoped the Doctor would offer to share his bed with her but…
She makes her way to Leo’s Diner but the green mist infiltrates the place and freezes the staff and occupants. Blundering about inside the diner, she comes face to face with one of the spindly, pumpkin headed Hervoken. It reaches out to her.
The Doctor returns to Etta’s house. The cats have returned to normal so he makes his way to the fruit store in the cellar and discovers a secret steel door. It leads him to a tunnel.
Martha is a prisoner of the Hervoken; tied up in their chamber. Thanks to the TARDIS’s translation abilities she knows that the Hervoken are low on power and that the book (or Necris) is barely able to supplement their needs. One of their number dematerializes from the chamber.
On the baseball field Rick, Scott and Thad are helping the adults set up the carnival. Taking a break, the boys walk past Mr. Everson. They are barely out of sight (not far in the thickening mist0 when they hear him shout out. Running back they find that he has vanished, leaving only his hammer as evidence that he was ever there.
In the chamber, Martha sees the Hervoken return with Mr. Everson. The terrified man is fed into a hole that opens like a mouth in the wall. His screams are cut off by the unmistakable sound of bones being crunched.
The tunnel leads the Doctor to the Hervoken chamber. He knows them to be a callous and pragmatic race so when he sees Martha he makes her rescue a priority. He uses a safety pin to prick his thumb and smears some blood onto one of the tendrils that litter the cavern. Almost immediately all of the plants, vines and creepers that are the Hervoken’s technology go into spasm, as do the aliens. His Time Lord blood is too rich for them to process. The Doctor frees Martha, grabs the Necris and flees.
Reaching Etta’s house, he races to the kitchen and pours salt into the book then rubs more into the cover and spine to stop the Hervoken being able to get the book back before he gets to the TARDIS with it He tells Martha that the book is the Hervoken’s starter motor and that the Blackwood tree is the tip of their buried space craft.
Unfortunately there is a green, crackling force field around the TARDIS that keeps changing frequency even as the Doctor changes the setting on his screwdriver. The Necris is becoming soft and slimy as it tries to break through the salt barrier the Doctor put on it.
Rick, Scott and Thad fail to convince Mr. Pirelli of Everson’s mysterious disappearance and are wondering where the Doctor is – he seems to be their only believer – when the Time Lord emerges from the mist demanding some iron. Rick gives him Everson’s hammer and the Doctor uses the sonic screwdriver to make the metal pliable and fashions a band that he uses to wrap around the Necris. In response to a question from Martha he tells them that the Hervoken have the ability to transform matter, alter perception and to shift time. They are an ancient race who became involved in a war with the Carrionites. The ship under the town crashed thousands of years earlier and the Hervoken used psychic energy to lure settlers who would build a town which the Hervoken could use to induce inhabitants to help them; building tunnels such as the one from Etta’s fruit store. The digging up of the Necris was planned, too, coinciding with the ship reaching readiness to leave. The problem is that the fuel they will require is negative emotional energy so they are likely to cause a mass atrocity at the Halloween Carnival to extract pain, terror and distress from the citizens. The Hervoken are too physically weak to perform this themselves so they will use agents (like the leaves and cats) to commit the atrocity for them.
The Doctor discounts alternatives: they will be ready for it if he tries to use his blood on them again; he can’t give them an alternative fuel source because the ship’s launch would destroy the town; the Necris cannot be destroyed. The only solution is to talk to them. Entrusting the book to Martha he steps out into the mist.
Jim Tozier’s lunch is disturbed when a psychic pulse urges him to dress in one of his own costumes – the evil clown. Once he has it on he finds that he cannot remove it. Suddenly a new personality inhabits his mind. As Martha and the boys pass the front of his shop he leaps through the plate glass window. Thad and Scott flee in terror but the clown chases Martha and Rick through the town. Eventually they find a girl’s skipping rope and use it to trip the clown. Martha beats it around the head with the Necris, causing it to revert to a man in a mask. Tozier has no recollection of these events.
In their chamber the Hervoken listen to the Doctor’s offer of a lift in his TARDIS before laughing and tying him up.
Rick takes Martha to his house: his mother is not pleased to see Rick’s friend and her mood worsens when they try to explain what has been happening. She only begins to come round when Chris interrupts to support them. Their conversation is disturbed when rubber bats from their Halloween display come alive and attack them. They race to the bathroom but a skeleton breaks down the door. Chris produces a lighter and burns it to the ground (it was made of cardboard).
By three o’clock Etta is worried by the non-appearance of the Doctor and resolves to take matters into her own hands. She goes down through the cellar to the Hervoken chamber. In the chamber the Doctor is telling the Hervoken that the rest of their people have been banished to the darkness by t he Eternals for their endless war with the Carrionites. Even if they launch their ship the Eternals will catch them. They merely ignore him. However, they also ignore Etta who passes through them as if she was not there. She has absorbed so much of their energy over the years that they think she is part of their technology. She genuinely has the power to think things into happening and the Doctor encourages her to use her mental powers to release him.
As they escape the cavern the Hervoken raise the alarm but when the Doctor threatens to open his arm and spill blood onto their tendrils they dismissively let him leave.
He returns to the Pirelli house and meets up with Martha. For several hours he sits alone with the Necris until she finds him and they head off to the carnival. After a while the mist begins to spin into a vortex. Tendrils of green light reach down from the vortex and strike the children at the carnival transforming them into the witches, werewolves and ghouls that their costumes depict. They begin to pursue their parents with murderous intent. Thad assaults Rick and Chris but Martha saves them by beating him with a large book and the Doctor sedates him with a Lepscillian massage technique.
As Hervoken begin to materialize around them and the mayhem continues the Doctor holds the Necris above his head and demands they stop or he will use his screwdriver to destroy it. An evil sprite flies in from behind him and rips the book from his hand. The Hervoken leader performs a gesture of triumph as the book dematerializes and reappears in its place in the aliens’ chamber. The transformed children close in on the Doctor and Martha.
In the chamber the Necris glows fiercely and then a whine of unbearable pain fills the Hervoken ship. The children at the carnival return to normal, the Hervoken thrash about and then crumble to dust and the green mist fades away. A subterranean rumble is followed by a bright flash in the night sky that disintegrates. Martha recognizes it as the Hervoken ship. She asks the Doctor what he did to the book before he allowed it to be stolen and he replies, ‘Never underestimate the power of the printed word. End of story.’
There is a crater at the bottom of the Pirelli garden; the black tree is gone. The Doctor recounts that he subverted the kinetic energy of the Necris which caused the ship to implode. He had to make a show of defending the book or the Hervoken would have been suspicious if he had merely returned it to them. Mr. Pirelli demands to know who the Doctor is but he is interrupted by Etta Helligan bearing a plate of cookies. The Doctor and Martha slip away back to the TARDIS.