The Icarus Girl tells the story of eight-year-old Jessamy Harrison (Jess). She is a mulatto from an English father and a Nigerian mother. When the novel opens, the Harrisons live in England. Jess lives a solitary life and has no friends. After she has shut herself in a cupboard for almost half a day, her mother is puzzled and proposes that they visit their Nigerian relatives, thinking that this may permit Jess to open-up a little and make friends. Jess is particularly excited about visiting Nigeria, especially as it is her first time to go there.
Upon arrival, Jess has a strange feeling about Nigeria. Things are not the way she expected. Everything is different from what she knows in England. Nigeria seems too ugly and too hot for her. Her mother’s family, according to her, behaves in a very strange way. At the beginning, she has difficulties in accommodating most of their traditions and customs, especially as her grandfather calls her Wuraola (meaning “gold”), a Nigerian name, instead of Jess. When they finally get to know each other, they start feeling at ease.
One day, Jess decides to visit her grandfather’s Boys’ Quarters. She is surprised and frightened when she sees a “HEllO JEssY” written on the surface of the tabletop in the dusty Boys’ Quarter. Nobody has ever called her like that before. It becomes evident to Jess that someone lives there, observes her permanently and switches the light at the Boys’ Quarter on and off in the night. This incident seems particularly strange to Jess because her aunt earlier told her that the Boys’ Quarter is abandoned and nobody lives there. She however does not talk of this to anybody.
Later, Jess meets a strange girl named Titiola. The latter repeats the same “Hello Jessy!” that was inscribed in the Boys’ Quarter. Jess concludes that Titiola (Tilly Tilly) is the mysterious habitant of the Boys’ Quarter. At first, Titiola just repeats exactly what Jess says when she attempts to have a conversation with her. Thereafter, she starts a real conversation and even proposes friendship.
With time, Jess and Tilly became good friends. However, Tilly Tilly seems to be a supernatural creature since she is able to pass through locked doors and appear wherever she wants to. She becomes very close to Jess, to the extent of knowing all what is going on in her life, right from England.
The time comes when Jess’ family moves back to England. The problem of solitude she had before moving to Nigeria seems to have gotten worse. Doctors are inept since they cannot detect what Jess is suffering from and she cannot express how she feels.
One day, Tilly Tilly mysteriously appears in Jess’ school, pretending that her family has just moved to England. With time, Jess notices that she alone can talk to and see Tilly Tilly. The latter starts behaving in a strange way, destroying things and revealing secrets, one of which is the fact that Jess once had a twin sister, Fern, who died as a child.
Oyeyemi tells us that twins are supernatural beings in the Yoruba tradition. If one dies, the family has to make some rituals and sacrifices to Ibeji, the god of twins. Otherwise, the dead twin will keep on tormenting the living one. Jess’ grandmother is particularly troubled by this situation since she remembers that the sacrifices were not made after Fern’s death.
Jess’ parents take her to a psychiatrist, Dr. McKenzie who has a daughter named Siobhan (Shivs). Jess’ friendship with Shivs makes Tilly jealous. The latter therefore decides to deal with Shivs by injuring her in a mysterious fall down a flight of stairs. In the course of reading, we notice that from time to time, Tilly takes control of Jess’ body. This situation alarms Jess’ family. They all rush back to Nigeria for the sacrifices to be made to Ibeji. It seems to be too late for this to be done. Jess’ grandfather proposes to take the child to a traditional Doctor but Daniel, Jess’ father, refuses. To escape from this dilemma, Sarah tries to run away with her child but they have an accident in which Jess is injured and hospitalized.
The novel ends with Jess’ grandfather bringing an Ibeji statue to the hospital in Jess’ room, to reclaim Jess’ body from Tilly Tilly’ control. There is some sort of fight between Tilly and Jess over who will take final control of Jess’ body.