Joe Coppock is a sickly boy who lives in an old timber-framed house in a rural English valley. He has amblyopia, a condition known as "lazy eye," and wears a patch over his good eye to force the weaker one to strengthen. He spends his days reading comics on a mattress atop his chimney cupboard, marking time by the midday train he calls Noony.
One day, Joe hears the cry of a rag-and-bone man. In the yard sits a creased-faced man with green-violet eyes on a cart pulled by a white pony. Joe offers him old, unwashed pyjamas and a lamb's shoulder blade he had collected. From a chest packed with elaborate crockery, Joe chooses the smallest item: a chipped white jar labeled "Poor Mans Friend." He is stunned to find his own name engraved on the chest's brass plate. The man also gives him a donkey stone, a rough grey scouring block used to whiten doorsteps, with a horse carved on one side. They go inside and sit in the large open chimney. Three heavy bangs sound on the door, though no one is outside. When Joe lifts the latch, a blast of winter wind and darkness bursts in, a shrieking sheet of night that escapes under the eaves. Only then does the man reveal his name: Treacle Walker.
Treacle Walker declares himself a healer of all things except jealousy. He produces a bone instrument, a hollowed human shin bone, and plays a haunting melody. When Joe tries it, the bone produces the call of a cuckoo, and a real cuckoo answers from across the valley. Treacle Walker tells Joe to whitewash the doorstep with the donkey stone to protect the house. As Joe wets the stone, he briefly glimpses his own name in silver letters around its edges. He whitens the entire step, and Treacle Walker departs.
The next day, Joe checks his museum, a small personal collection of found objects. The shoulder blade is gone, replaced by the donkey stone and the jar, confirming the exchange was real. Inside the jar he finds traces of a sticky green-violet paste. While adjusting his eye patch, his paste-stained finger touches the eyelid of his good eye, filling his head with jagged light and pain. At a subsequent eye examination, his good eye reads an entirely different set of letters that the optometrist insists do not exist on the chart.
Drawn by a cuckoo's call, Joe enters the alder bog by the brook. Though the copse is small, it becomes labyrinthine and endless. A naked, copper-brown man wearing only a leather hood sits up from the water and addresses Joe by name. This is Thin Amren, an ancient being who has slept in the bog for ages, and he explains that Joe's good eye now possesses the
glamourie, a magical second sight caused by the paste. Joe's weak eye sees ordinary reality while his good eye perceives an expanded, otherworldly layer. Thin Amren instructs Joe to stop wearing the patch and use both eyes, warning that what sees is also seen.
While reading his favorite comic,
Knockout, Joe watches a character named Stonehenge Kit the Ancient Brit pull himself alive from the panel onto Joe's mattress. Joe's hand passes through the paper into the comic's world. Two villains from the strip, Whizzy the Wicked Wizard and his henchman the Brit Basher, emerge from the comic onto the cupboard and vanish through the tall bedroom mirror, visible only as reflections. Joe's prized dobber, a large glass marble, falls into the comic's world and is lost.
Treacle Walker returns. He reads the paper from Joe's eye test and identifies the letters as Latin hexameters describing the donkey stone. He calls the chimney the
axis mundi, the heart of all that is. He returns the lost dobber from his bag, the Corr Bolg. Joe meets Thin Amren by the brook, and Thin Amren warns that what is out is in and what is in is out. Back home, Joe finds a
Knockout strip depicting his own house, with the villains repelled by the whitened step. Treacle Walker reasons that the step bars crossing in either direction: The villains entered through the comic and passed through the mirror, but the step now traps them in the mirror world. He tells Joe the burden is his alone, because Joe is the one who dreams.
In a layered dream, Joe follows silver hoof prints to a hillock and hears bone music underground. He returns home to find Treacle Walker sitting in the chimney across from Joe's own double. Both Joes scream. Treacle Walker separates them, forbids them to touch or look in each other's eyes, and plays the bone to resolve the crisis. Treacle Walker leaves behind a new
Knockout with entirely blank Stonehenge Kit panels. Joe tries to destroy the mirror without success. When he strikes it with the donkey stone, his hand passes through, dragging him into a reversed mirror world. He navigates successive mirrors, each passage growing colder, until Stonehenge Kit emerges to help. Kit takes Joe's old pyjamas as a decoy while Joe escapes with the
Knockout. Joe presses the open comic against the mirror glass, and the characters are forced back into the panels, which now depict the villains' defeat. Joe seals the mirror entirely with donkey-stone paste.
Treacle Walker says the crisis is done but not over. He offers to remove the
glamourie, but Joe leaves the question unanswered. Treacle Walker says it is time to make an end. At the bog, he has Joe sharpen five green alder branches and summons Thin Amren, who refuses to sleep and dream unless given Joe as a companion. Treacle Walker explains that Thin Amren dreams all that exists: If he will not dream, reality ceases. The sky cracks open and a colossal cuckoo descends, bringing consuming blackness. A speech bubble from Stonehenge Kit flashes across Joe's closed eyelids. Joe fires the dobber from his catapult, striking the bird and igniting a flash that restores light. He dips the donkey stone in bog water and rubs the cracks in the sky closed.
Joe walks onto the bog and finds Thin Amren old and emaciated. He fills the jar from a pool and gives Thin Amren a final drink. Weeping, Joe presses Thin Amren beneath the water and mud, then drives the five stakes into the bog, pinning Thin Amren to dream forever.
Back in the chimney, a speckled egg in the ashes becomes the dobber when Joe picks it up. He builds a fire using a crumpled
Knockout for kindling. He asks Treacle Walker whether he is dead. Treacle Walker answers that he would not say Joe is dead; rather, "in this world you have changed your life, and are got into another place" (149). Joe asks what Treacle Walker wants most for himself. Treacle Walker, who has never been asked, longs for oblivion, to hear no more the beat of time. Joe gathers the Corr Bolg with all his talismans inside, drops his eye patch into the fire, crosses the shining step, and jumps onto the cart. It is midday. Noony rattles past. Joe calls out Treacle Walker's own cry, offering rags, pots, and donkey stones. He has taken up Treacle Walker's itinerant healer's role, releasing Treacle Walker into the oblivion he longed for.