Detective Superintendent Pierre Niemans, is sent to the small university town of Guernon in the French Alps to investigate a brutal murder.

Dr Cherneze, explains that the school's isolation led to inbreeding amongst the professors, with increasingly serious genetic disorders.

Coincidentally Detective Inspector Max Kerkerian is in the nearby town of Sarzac investigating the desecration of the grave of Judith Herault, a girl who died in 1982, and the theft of her photos from the local primary school.

The mother tells Kerkerian that at the faculty's hospital in Guernon, they were attacked by "demons" on their way back and when they fled, her husband and her daughter were killed in the road accident.

Sertys, they deduce, must have swapped Fanny for one of the dead faculty babies while leaving her identical twin, Judith, with her birth family as a control subject.

[1] The Crimson Rivers, a tv series following the novel and its adaption first aired in 2018, starring Olivier Marchal and Erika Sainte.