After the brutal death of her mother, Opaline is taken to the Onirium. What is this strange place, where mysteries accumulate and whose doors open only to enter? Atrocious practices are at work there. Every night, a machine materializes the dreams of children to put them in bottles. It is said that their splendor is used for the city's poor.
Opaline struggles to get used to the rigors of the orphanage. Between the bad blows of her friends and the perversity of the director, daily life is difficult. At the same time, the doubts about the death of his mother intensify. She seems to have been murdered... Except that in Chronopolis, the perfect city, murderers do not exist.
Opaline's investigation will take her through the winter and the forbidden, to get closer to the dark secrets of the Empire.