Joseph Coppock, also known as Joe, is a young boy who lives in a house by himself somewhere in the English countryside. Although we never learn where exactly he lives, the house is located near places called Big Meadow, Barn Croft, and Pool Field. Every day at noon, a train that Joe has nicknamed "Noony" passes through, and Joe uses it to tell the time. Joe also wears an eye patch in order to correct his lazy eye.
As Joe lies in bed one day reading his favorite comic book, Knockout, he hears a call come from outside his window. A rag and bone man (a peddler of discarded items) arrives at his house and proposes that Joe give him a rag and a bone in exchange for a "pot and a stone." After Joe gives the man a pair of pajamas and a lamb's bone, the man gives him a stone with an engraving of a horse on it, as well as an old jar. The man tells Joe that his name is "Treacle Walker," referring to the antiquated definition of "treacle" as medicine. Treacle Walker tells Joe that he is a healer who can heal everything but jealousy.
After Treacle Walker leaves Joe, Joe starts to realize that his vision is changing; all of a sudden, he is able to see things with his good eye that aren't there when he uses only his lazy eye. Using this second sight, Joe ventures out past the Big Meadow and meets a man named Thin Amren. Thin Amren lives in a bog and tells Joe that he has gained the "glamourie" in his good eye—a magical power.
Treacle Walker continues to visit Joe and Joe, frustrated with the glamourie, demands that Treacle Walker help him get rid of it. However, Treacle Walker elusively refuses, instead speaking in his usual riddles and wordplay. Joe continues to try and go about his routine, reading comics, but soon finds that even the characters from his Knockout comic books are coming to life and entering his world.
Soon, Joe embraces his second sight, traveling between the two realities through a mirror and even seeing a second version of himself reflected in the alternate reality. As the novel comes to a close, Joe returns to the bog and faces off with a large cuckoo, who he must fight in order to reach Thin Amren and force him back to sleep beneath the bog's surface. After he succeeds, Joe returns to his home and meets Treacle Walker once more. Joe discovers he has gained the ability to use the "Word," which allows him to control Treacle Walker's pony. Joe boards Treacle Walker's cart and rides off into the sunset, taking Treacle Walker's place as the new rag and bone man.
