On the way from Marlborough to Cirencester, a carnival is forced to a stop by a little girl standing in the road. She wears a sign that marks her as “Leonie Pickering”, and although a few members of the troupe attempt to approach her, she snarls and hides under one of their trailers like an animal. Only Piet, a dwarf who operates the funhouse, manages to connect with her, providing her with clean clothes, food, and shelter. Marek, the carnival’s knife-thrower, finds Leonie mysterious and abnormal. He wonders if Piet took her in because she reminds him of his dead sister or, worryingly, if he wants a sexual relationship with her. At first, Leonie remains silent for a few days. Afterwards, she talks constantly. Sometimes, she makes fun of Piet, but she also runs towards him whenever she feels threatened, creating the impression that she is devoted to him. Several more days pass, and it becomes clear Piet is in love with Leonie. He asserts she is a talented acrobat, and although Marek attempts to disregard this, even he cannot deny it.

One day, Marek stumbles upon Leonie’s secret training session with the horse-riding acrobat Milena. She tells him Leonie is a natural—unlike any other she has seen before. Soon, Leonie begins performing with the troupe, even overtaking Milena’s act in popularity and skill level. However, Leonie has been carelessly messy, refusing to clean up her things and letting Piet do it for her instead. When Marek tells Piet she should help more, Piet dismisses his advice, saying a person is only young once. Marek also realizes Piet looks rather ill and Leonie, no matter how much food she eats, remains thin and gangly. In the thrall of his growing obsession for the girl, Marek follows her as she leaves the troupe’s campfire one night. He forces himself on her with a kiss, but draws back at her sour and unpleasant taste. When he asks her what is wrong with her, she laughs and runs away, leaving Marek wondering if he might catch whatever disease Leonie has.

When winter comes and the carnival’s season ends, Piet leaves for Norfolk and takes Leonie with him. Marek grows restless with his obsession with Leonie and decides to travel to Cirencester for a few days, leaving his partner Mae behind. He walks down the streets and past the houses, examining the places where Leonie may have lived. When it starts to rain, Marek shelters in a library, paying for an hour to spend on one of its computers. He begins researching Leonie, eventually coming across a local Cirencester newspaper that discusses the murder trial of a Wilfred Pickering. Wilfred had been tried for the murder of his grandson, but was acquitted. The newspaper includes a photo of him leaving the courthouse with his daughter Willis Quayle. Despite continuing his research on the Pickerings, Marek finds little else and gives up for the day. He visits a pub for dinner, reading a book from one of its shelves while he waits for food. The book details the various legends of Gloucestershire, including the Lammas Worm. According to the book, some villages formed a cult for the worm, offering it young girls as reproductive vessels. After the girls produced a man-worm hybrid child, they were killed to end their suffering. While unsettled, Marek still eats, and soon two men sit at the table next to him. They discuss how the authorities finally caught the “Pickering lunatic”, as well as how the man drowned his deformed grandson to put him out of his misery and how he supposedly reproduced with his own daughter.

As Marek is finishing his food and leaving the pub, he realizes he has caught a cold. He almost passes out en route to his home with Mae. Over the next few months, his work and everyday routine around the house and with Mae seems to soothe his anxiety. When spring comes, he dreads the carnival’s upcoming season. At the start of the season, Piet excitedly tells Marek and Mae that Leonie and he have married, then mentions to Marek that Leonie is pregnant. Nauseous, Marek starts watching Leonie for signs of what she may really be. However, his sexual desire for her still grows. Soon, the carnival reaches Cirencester again. One evening in the town, a boy approaches Leonie. He looks like her, skinny and gangly. The boy tries to grab her and take her away, saying their father misses her. She screams and fights back, and when the adults scare him off, Leonie runs to Piet for safety. She tells him the boy is her brother, Aaron.

Leonie does not perform for nearly a week. When she emerges from Piet’s trailer, she is disheveled and appears unwell. Though her illness seems to go away when she finally performs, Marek runs into her one night after a show, witnessing her scuttling outside in the dark. She disappears quickly into Piet’s trailer, and Piet confesses to Marek that he knows Leonie must be seeing Aaron. Mae begins worrying for Marek’s state of mind, suspecting he is not telling her something is bothering him. At night, the two are awoken by Leonie’s scream. Aaron had snuck into Piet’s trailer, naked, and laid atop the girl. After the boy is chased off, Leonie locks herself in the trailer. A couple days pass, and Marek and another of the troupe’s men break into Piet’s trailer to look for Leonie. When they finally find her, she bolts away into the woods, her previously flat stomach seemingly engorged and swaying with some fluid. A chase ensues, but she escapes into the night. The next day, Marek and a couple other troupe members find her dead body near a poacher’s shack, but not the child she must have been carrying. In spite of this, Marek sees a trail of brittle, clear, solidified slime near her body. He crushes it to hide its trace and realizes today is the first of August, or Lammas day.

Soon, Piet falls ill and dies in a hospital. Marek and Mae complete the carnival season, then separate from the troupe, pursuing new careers. Marek becomes a sales representative for a pharmaceutical company and spends his days traveling through England. On one trip, he happens to drive past the place where the troupe found Leonie’s corpse. He tries to find lodging, but all of the inns have no vacancies, their rooms filled with tourists who have come for the Lammas fair. Marek walks through the forest to the shack, but the area around the small building and the building itself have been decorated. The trees have been hung with ribbons, the shack’s walls have been written on in strange symbols, and Lammas bread has been set at the doorway. Marek then finds an ornament hanging near him. It is clear and star-shaped, containing the long, segmented body of an insect. With its legs splayed open in frozen movement, it seems to have died trying to escape.