Helen Oyeyemi's first novel explores the world of eight-year old Jessamy Harrison, who is miserably, decidedly different than her peers at school in London. Her mother, Sarah (or Bisi) is Nigerian, and her father, Daniel, is English. She is prone to horrible fevers, and she is bookish. She is shunned and scorned by popular girls, and even by her cousin Dulcie. But when a wild, magical force called TillyTilly reveals that Jess had a stillborn twin sister, Jess's torment becomes far more complicated.

TillyTilly appears, at first, to be a wonderful friend, and an antidote to Jess's constant loneliness, but the reader is wary from her very entrance into the novel. She writes a message in the dust of the abandoned Boys' Quarters of Jessamy's grandfather's compound in Nigeria, when the Harrison family goes for a visit, so that from their initial encounter, TillyTilly is drawing Jess into a place where she should not be. TillyTilly only appears to Jess, which automatically makes an already-vulnerable Jess look like her imagination has reached dangerous excess. Their adventures, which are fun at first Ð spending a day in a closed amusement park all by themselves, and using the rides as they please Ð quickly become terrifying. Oyeyemi does a wonderful job characterizing TillyTilly as a temptress and a captor; the well-drawn Jessamy, who is eager to be liked and included in friendship, is both endearing and perceptive, so that the reader does not feel trapped in an eight-year-old's world. There are times when there is copious detail about the mundane life of a child Ð especially when Jess befriends Siobhan McKenzie, her therapist's daughter, and a series of play dates and sleepovers ensue. These are the weaker moments in the novel, when games and giddy conversations are described, but the story does not truly develop.

Far more interesting is the concept of "three worlds", which exists both explicitly and implicitly in this tale. When TillyTilly haunts Jess about Fern by placing a wailing baby under her bed, and Jess finally asks her mother, " '...Was there two of me?'", Sarah tells Daniel: "Jess lives in three worlds. She lives in this world, and she lives in the spirit world, and she lives in the Bush" ("The Bush," we later learn, is "A wilderness. A wilderness for the mind"). While Jess's severed-twin identity provides for a multiplicity of worlds, so does the literal life she leads, in which her parents offer her two strikingly different cultures (and disciplinary methods). Then there is the literary world, of which Jess is deeply enamored, and in which she encloses herself, without friends, until TillyTilly's arrival. One reads on because there are times when Jess, an anxious and kind-hearted child, makes the wrong decision simply because TillyTilly has asked her to; other times, alarmingly, TillyTilly demands it, and leaves her no choice. Although Oyeyemi frees Jess from TillyTilly's grasp at the very end, throughout the majority of the novel, Jess has the difficult, albeit thrilling, challenge of struggling against a nemesis that no one else can see.