Trying to find inspiration in his mother's native village, author of detective novels David Rousseau finds himself investigating a most peculiar serial killer.
David Rousseau, writer of detective novels lacking inspiration, returns to Mouthe, a village in the Jura where he spent part of his childhood, reputed to be the coldest in France. A monk, Brother Giacomo, promised to tell him a secret about his mother. On his arrival, David learns that the monk in question is dead, while strange events disrupt the frozen daily life of the village: the discovery of a human ear, then that of a corpse imitating a self-portrait of Van Gogh. To the despair of the local gendarme, Warrant Officer Louvetot, David interferes in the investigation by constructing hypotheses worthy of his novels. Could a serial killer be hiding... in Mouthe?
Disturbed by the clues collected about his origins, David is decided to stay in Mouthe. Especially since a new crime scene reproducing a famous painting has been discovered... But Warrant Officer Louvetot, deaf to his intuitions, orders him to no longer hinder the investigation. David finds a better ally in Nicki, the singer from the bar Les Flocons, who is immediately seduced by his fantasy. Their suspicions fall on the village librarian, a young man with obsessive behavior and a keen interest in art history. The assassin strikes again.
Louvetot begins to believe that David's hypotheses are not so far-fetched. He in turn follows the trail of the media library, which leads him, like David, to the monastery. But their suspect turns out to be innocent and Louvetot is removed from the investigation. It is then that the killer makes contact with him... Meanwhile, Aurélie Poulidor, the French teacher who admires David Rousseau so much, collects the latest writing work from her students. One of them is particularly striking.
Now partners in their clandestine investigation, David and Louvetot saw the killer up close. They also saved Aurélie Poulidor at the last minute... David, one of whose novels was found at the last crime scene, begins to wonder if the murders are not linked to the personal affair that led him to Mouthe . But the assassin continues to play with them, sowing mysterious clues. To understand them, Nicki encourages David to delve into his imagination.
David meets Aurélie Poulidor for a tense interview, during which she reveals her astonishing secret to him. Meanwhile, Louvetot and his colleagues are making progress. A DNA analysis made it possible to identify their suspect: Ulrich Becherel, a convict obsessed with amputations. The testimonies of Brother Auguste and the village disguise seller bring them even closer to the truth. Now Nicki is under threat...
David and Louvetot set out in pursuit of Nicki's kidnappers and Lyès, the adjutant's companion. This ultimate treasure hunt leads them to a forest where an old hermit nicknamed the Indian lives, who is none other than Nicki's father. David recognizes him... Not far away is the monastery where it all began. There, a final work composed for them awaits them.
