This novel is the middle volume of a trilogy together with Genesis and Lullaby. It is set in what reminds one of a mediaeval walled city, under the rule of a powerful and oppressive church, but with some modern technology (such as cars). In this two class society the group of the “people of the night” are suppressed, as they allegedly lack a soul. They only enter the city at night, do manual labour and look for scraps of food to eat.
In this novel, the teenagers Tristan and Grace, badly injured and in pain, tell each other their stories after their car crashed and partly tumbled down a cliff, hanging upside down, in the dark. Tristan and Grace do not know if anyone will be able to see and rescue them in daylight nor if they will survive or not. The facts that they are hanging upside down, and the ways in which their worlds were turned upside down, are nicely reflected in the way the book is printed: the entry pages are upside down and you need to turn the pages from right to left.
Tristan used to attend a boys’ church school, St Augustine’s, in which the students are trained in philosophical enquiry through interrogation classes. Because of his intelligence and strong will, the rector decides to use him in a cruel experiment regarding the nature of free will. He asks Tristan to draw a sketch of a girl of the people of the night. Tristan is intrigued by the beauty of the girl’s body and, seeing the fear in her eyes, realises that the church has been lying about the lack of souls in the people of the night. Another brother, sent by the rector, takes Tristan into town at night where he witnesses a passing, i.e. a ceremony of the people of the night in which mothers give the dead bodies of their babies away, not having access to any of the rituals and consolations the church offers its followers. One of the despairing mothers is consoled by a young woman (whom he later finds out to be Grace). They look at each other, Grace thinking he is an angel and Tristan falling in love with her.
As a punishment for leaving school at night, Tristan is locked away in a room in the monastery for two years and has to undergo a number of tests and experiments. The rector has told him that he wishes to use Tristan to prove that every action is predictable, thus there is no free will. Tristan spends the two years training himself through meditation to choose not the choice he would normally take, but another one.
After two years, he faces the final challenge. He and two boys of the night are put into the same room and instructed to complete a complicated puzzle. Whichever boy puts in the very last piece of the puzzle is the winner. The prizes are highly motivating for all three: Tristan’s prize is that he will be brought to Grace and given papers to leave the city, the other two boys would get the chance to escape their lives as people of the night. Tristan wins and is brought to Grace’s house. Looking through her window, he sees her with another man. Terribly upset, he leaves the city without her for the heathen settlements, where he lives with a group of homeless until their leader dies, who is Tristan’s friend.
Grace tells her story of growing up in a convent, losing her best friend (and blaming herself for it) and helping the people of the night at their passing ceremony until she is discovered and expelled from the convent. She would have died outside the convent, alone and helpless in the freezing cold City, had not Mary, one of the women whom she had consoled after her child died, and her husband Anthony, taken her in. In order to help pay for her keep, Grace starts to work as a prostitute. When she is alone in the house with Anthony, he expects to have sex with her and she flees. Before Anthony entered her room that night, she noticed Tristan in the street, looking up at her window, and thought her angel had returned and wanted her to follow him. However, when she flees the house after the encounter with Anthony, Tristan is gone. Grace leaves the city for the heathen settlements, just like Tristan, and it so happens that Tristan sees her there. He steals a car, picks up Grace and causes the terrible car crash.
Eventually, when dawn breaks, Tristan confesses to Grace that he is so obsessed with proving the rector wrong and demonstrating the existence of free will that he wants to do the opposite of what he really wants to do and kill Grace. He starts to choke her, but she proves to him the pointlessness of his argumentation. In the end, in a final and decisive act of free will, the two of them rock the car to make it fall down the cliff further, hoping to be more visible to possible rescuers, but also risking death.