Jessamy Harrison is an eight-year-old girl living in a small town in England. Her father is English, and her mother is Nigerian. Jess is a smart, introverted child who likes to spend her time reading books and playing inside. Jess’s parents take her on a trip to Nigeria to visit her extended family. While staying at her grandfather’s compound, Jess sees strange shadows in the windows of the Boys’ Quarters, a building that was once the servant’s quarters, but now stands abandoned. Soon, Jess is visited by a girl her age named Titiola, whom Jess nicknames TillyTilly. At TillyTilly’s encouragement, Jess and TillyTilly sneak into Jess’s grandfather’s study at night and to a nearby amusement park, where TillyTilly magically unlocks the gates and make the rides work on their own.

Once she returns to England, Jess starts school. Jess is very smart and is placed a grade ahead, but she has trouble fitting in with her classmates, often throwing tantrums in class. Eventually, TillyTilly appears, saying her family recently moved to Jess’s town. TillyTilly helps Jess sneak into the home of Jess’s classmate, Colleen, who is especially mean to Jess. Jess and TillyTilly are invisible and able to move around Colleen’s house without anyone seeing them.

As TillyTilly and Jess’s friendship grows, Jess continues to struggle socially at school. Jess’s parents take Jess to talk to a psychologist, Dr. McKenzie. Jess is reluctant to speak to Dr. McKenzie but eventually warms up to him. Jess also meets Dr. McKenzie’s daughter, Siobhan, and the two become friends.

TillyTilly often visits Jess in her bedroom at night. One night, TillyTilly reveals that Jess used to have a twin sister named Fern, but Fern died as a baby. When Jess wakes up, she asks her mother about Fern’s death, and her mother confesses that it is true. Jess’s mother is upset, unable to understand how Jess knew about Fern. Jess overhears her mother tell her father that they should have done an ibeji carving for Fern, a small wooden statue made for children who died young. Jess’s mother explains that twins live in three worlds: this world, the spirit world, and the Bush.

Over time, TillyTilly’s actions become more troublesome and mischievous. TillyTilly promises to get anyone who hurts Jess. At school, Jess gets in trouble for cutting pictures out of school books. A few days later, Jess’s class has a substitute teacher, and it is unclear if or when Jess’s teacher will return. Jess is sure that TillyTilly did something to hurt Jess’s teacher. Another day, TillyTilly appears in the bathroom mirror. TillyTilly begs Jess to switch places with her, and shatters the glass, causing Jess to scream. As Jess is screaming at her parents, her father slaps her. The next day, Jess’s father collapses at work and is hospitalized. Jess is sure that TillyTilly made her father ill.

TillyTilly becomes jealous of Siobhan, claiming that Jess likes Siobhan more than her. Jess tells Siobhan about TillyTilly. Later, Siobhan tells her father, Dr. McKenzie, about TillyTilly even though she promised Jess she wouldn’t tell. One night, while Siobhan is sleeping over at Jess’s, she falls down the stairs and badly cuts her neck on the banister. Jess believes that TillyTilly hurt Siobhan because Siobhan broke her promise to Jess. It is unclear whether Jess pushed Siobhan or TillyTilly caused the injury.

Jess’s parents take her back to Nigeria for her ninth birthday. When Jess arrives at her grandfather’s house, he gives her an ibeji statue to represent Fern. Jess is disturbed by the statue and asks her grandfather why he never told her about Fern.

The day of Jess’s birthday, her family sets up a party outside. Jess notices TillyTilly under one of the tables. TillyTilly grabs Jess’s ankle, causing both girls to fall over, and Jess to feel as if she is floating away. When Jess gets up, she seems different, more cheerful and suddenly able to speak Yoruba. Jess’s grandfather wants to take Jess to a witch doctor, causing him and Jess’s father to get into a fight. Jess’s mother decides to take Jess to stay with a friend in Lagos for a few days, but on the drive to Lagos, they get into a car accident. Jess ends up in the hospital, unconscious.

The novel ends with a dreamlike scene in the Bush. Jess travels through the Bush and comes face-to-face with Fern. Finally, Jess forces her way back into her own body, which had been taken over by TillyTilly. It is unclear if TillyTilly was ever real or a figment of Jess’s imagination.