Between 1943 and 2003 nine people have been stabbed to death with a most unusual weapon: a trident. In each case, arrests were made, suspects confessed their crimes and were sentenced to life in prison. One slightly worrying detail: each presumed murderer lost consciousness during the night of the crime and has no recollection of it.

Commissaire Adamsberg is convinced all the murders are the work of one person, the terrifying Judge Fulgence. Years before, Adamsberg's own brother had been the principal suspect in a similar case and avoided prison only thanks to Adamsberg's help.

History repeats itself when Adamsberg, who is temporarily based in Quebec for a training mission, is accused of having savagely murdered a young woman he had met. In order to prove his innocence, Adamsberg must go on the run from the Canadian police and find Judge Fulgence.