Penny and Primrose are two girls who are evacuated with a group of children to a mansion in the English countryside during World War II. They are evacuated to escape the Nazi bombing of London (i.e., the Blitz), which took place from 1940-41. Penny is tall, thin, pale, and possibly older than Primrose, who is plump with curly blond hair. They become friends on the train, discussing their bewilderment over their evacuation, wondering “whether it was a sort of holiday or a sort of punishment.”
The girls arrive, along with a group of many other children, at the mansion—a big, eerie place surrounded by a forest. They are anxious and scared, thinking of themselves as orphans. Some of the children cry themselves to sleep that first night. The next morning, after breakfast, Penny and Primrose decide to explore the forest. A younger child, Alys, wants to go with them, but they tell her no, saying she is too little. Creeping into the forest, the two girls hear a “crunching, a crackling, a crushing, a heavy thumping, combining with threshing and thrashing,” plus a host of other noises. They also smell a stench like that of “maggoty things at the bottom of untended dustbins, blocked drains, mixed with the smell of bad eggs, and of rotten carpets and ancient polluted bedding.” Finally, they see a giant, fleshy caterpillar-like creature trundling through the forest, crushing foliage in its path and wailing terribly as it passes.
When the thing is gone, the frightened girls return to the mansion. The next day, they are sent to stay in separate places for the rest of the evacuation. They can’t forget what they saw, though they don’t discuss it with anyone. After the evacuation, the girls return to their families, which the war has altered. Primrose’s father has been killed on a troop carrier in the Far East, and afterwards her mother remarries, having five more children. Penny’s father dies in a fire in London. The years pass, and Penny goes to university, studying developmental psychology. Primrose struggles in school, due to having to babysit her younger siblings. Penny becomes a child psychologist, while Primrose holds a series of odd jobs before settling down as a children’s storyteller.
In 1984, Penny and Primrose, having had no contact during the forty years since they saw the thing in the forest, travel separately to the country mansion, which has been turned into a museum. They run into each other while looking at an old book on display. The book tells the story of the Loathly Worm, a giant creature that, according to legend, had once terrorized the countryside around the mansion.
Delighting at their reunion, the women have tea and talk about their lives. Neither is married, and neither has children. Finally, they discuss the day they met the loathly worm in the forest. “Did you ever wonder,” Primrose asks, “if we really saw it?” Penny replies, “Never for a moment.” They discuss the horror of that day, and how their lives have been affected. They wonder what happened to Alys, the child who had wanted to go with them into the forest, and agree that the worm must have killed her. After the discussion, the women feel better, realizing that they aren’t crazy.
Penny and Primrose agree to have dinner together the next evening, but neither of them shows up. On the following day, they set out separately for the forest surrounding the mansion. Primrose hikes for a while, then sits on a tree trunk, thinking of her mother, who used to make stuffed animals to give to her. Primrose hadn’t realized the animals were handmade, and when she eventually found this out, she was disappointed. Standing up, she resumes walking, telling herself a story about “staunch Primrose” (herself) bravely walking through the forest. She stops again, remembering more about her dead father and her “sniveling” mother with her “dripping nose.” She considers the difference between reality and imagination, and decides that the imagination is, to her, more real than reality. Finally, she walks out of the forest.
Penny is in a different part of the forest, trying to find the spot where she and Primrose had seen the loathly worm as children. She finds evidence of the worm: “odd sausage-shaped tubes of membrane, containing fragments of hair and bone and other inanimate stuffs.” Finding a spot to sit down, Penny reflects on her career as a psychologist, realizing that her encounter with the worm all those years ago “had led her to deal professionally in dreams.” She hears a rumbling and thinks it is the worm returning, but she sees nothing. She thinks about her own dead father. After a while, when night falls, she leaves the forest.
Penny and Primrose both return home, but Penny can’t stop thinking about the worm, so she travels back to the forest once more, deciding she needs to confront the worm. Finding the same spot, she waits and silently calls to the worm, which she then hears approaching. Penny is ready for whatever happens.
Primrose does not return to the forest, going instead to one of her storytelling sessions in a shopping mall. She smiles at her students and tells them about two little girls who saw, or believed they saw, a thing in a forest.