One day, 15 year old Petra is asked to drop off a parcel at Century Hall during her paper round and she meets old Edmund Century for the first time (she usually just leaves the parcel in the porch). Mr Century insists on giving Petra a gift, a copy of The Rubaiyat of Omar Khayyam, which is inscribed "To Eurydice, undying love - EC." She takes it home, being incapable of refusing a gift or of keeping an object even if it is unwanted. The next day, Mr Century's housekeeper bumps into Petra in the town and, unknown to Petra, drops an earring into her pocket. Later the same day when Petra is out riding with her friend Mel, her horse is spooked by something, and then as Mel is telling Petra that her GP father has been called out to see Mr Century, Petra sees Mr Century, lying wrapped in blankets with Dr Gaspard in attendance, and she notices an alcove in the room where she had talked with Mr Century. The alcove, which had been curtained the evening before, has a number of oddly-shaped bottles on its shelves along with a wooden pipe. As Petra is seeing the alcove (despite being some distance away from Century Hall), a belt of wind runs across the field and frightens her horse so that it bolts across the field, through a wood and into the grounds of Century Hall. The horse carries her to the ruins of a stable block that was destroyed many years earlier by a fire; but Petra sees people and horses in the courtyard, and sees and hears the fire raging. As they turn the corner of the building, the people and the horses disappear again.
That evening Petra wishes that "Edmund and father would be friends" just before she falls asleep, even though the two have never met. During the night Petra wakes and finds herself unable to recall her charm against the Darkling, a make-believe creature that is created from night time shadows on the wall of her room. The Darkling speaks to her and she knows it is calling to her, yet it calls her Eurydice. It becomes clear that Petra resembles Eurydice in her looks (being small, black haired and very pale), and through the gifts that Mr Century has given her, the Darkling manages to gain a foothold in the real world in order to work its mischief. Petra begins to dream of Eurydice, who died before she was born in the fire at Century Hall. Then Mrs Campbell lives a third gift for Petra, a white bottle that appears to be made from wax and which contains a scent that at one moment smells of Jasmine and at another smells foul. That night, whilst Petra is in bed, the Darkling talks to her, telling her that no one who dies is truly gone; Petra's mother died a few years earlier, stepping off the pavement into the path of a passing car. The Darkling speaks in Mr Century's voice, pleading with Petra to give it life because she has accepted its gifts. She refuses the Darkling's request and moments later a bough from the walnut tree outside her window (the one that casts the shadows which she named "The Darkling") crashes into her room, almost crushing her. Shortly after this the Darkling possesses Petra's father and he becomes angry and indifferent to his son and daughter, until he sees Petra apparently being attacked by his new boss, Graham Cooke, who made sexual advances towards Petra the night after her mother was killed. The Darkling goes beserk and uses Petra's father's body to attack Graham Cooke, who is killed.
There are hints throughout the story that Petra is a reincarnation of Eurydice, given the things she sees which aren't actually there, and that her "seeing" is a consequence of her acceptance of Mr Century's gifts. Mr Century and the Darkling also appear to be one, and they possess Petra's father after he inhales the scent that Mrs Campbell left at their house.