The book is the first volume of the series, combining the features of fantasy, detective story and dystopia. It describes the distant future in which humanity is subject to meticulous control and class segregation. The criterion for belonging to one of the classes is IQ, determined at birth according to measurements of synaptic density of the brain. Permits to have a child are issued by committees that severely assess the physical and mental health of future parents, and marriages between castes are prohibited.

The young librarian, Leeta finds herself accidentally at the center of political game and as the only witness to an attack on members of the global government she becomes a threat to many prominent figures. Her protector, Citizen Hakat, decides to send the girl to the moon, to an industrial city where she would be safer than on Earth.

The city of Lunnaria is completely dependent on supplies from Earth and theoretically falls under its jurisdiction, but it is really ruled by large mining corporations. Leeta gets assigned to the local police. Police schools have not existed for a long time, newbies are field trained by senior officers. The new job teaches Leeta how to be a guardian of order in a world dominated by economic and political struggle, and at the same time to fulfill the task assign ed to her by Citizen Hakat on her departure. She is helped by android Monty and Sue Herefort, the niece of Hakat and the specialist in acquiring data from the network.