""I loved this book... a mile away from a humdrum police procedural." "
A woman delivers an important letter to the mailbox and shortly after she is found with her wrists slit in her bath. An apparent suicide, were it not for a mysterious sign found near the body.
This mysterious sign encourages the local police to contact Commandant Adrien Danglard as the fountain of all arcane knowledge, and from him to Commissaire Adamsberg.
The letter takes him to a stud farm and from there to a group of people who had all been on an ill- fated holiday to Iceland ten years previously.
The farm also leads to a strange and secretive society for the Study of the Writings of Maximilian Robespierre. This is a collection of eccentric individuals with various reasons for an obsession with the revolutionary leader.
More murders, all with the accompanying strange symbol, are identified.