A dead body on the floor of the Bistro brings Gamache and his crew to Three Pines. No one claims to know who the dead man is. The investigation leads the team to a neighboring spa - recently opened in a house where previously a son murdered his mother. They find evidence that the body had been there - but it was not the murder site.  They find a cabin in the woods with a lot of blood, the home of the unknown corpse. The investigation continues - the man might have had a connection with the local Czech community - although no one claims to have known him. 
The ex-husband of the mother of the spa owner appears - a father that the spa owner thought was dead.  Nope - just too self absorbed to be bothered with his son. So after he wrote a book about his life working with autistic children, he went to India to study with a teacher, and was dead to his ex-wife. 
Gamache and Jean Guy talk to Gabri and Olivier, the Czech community, the Spa owners. They return to Olivier - especially after his fingerprints are found in the cabin. Tell the truth Olivier. He equivocates. Gamache travels to British Columbia to find the source of the inspiration for some wood carvings found in the cabin. Wood carvings with cryptic markings on the bottom.  Gamache learns that other carvings by the same person were sold on E-Bay. The spotlight returns to Olivier. Finally he admits that yes, he knew the hermit, yes he sold some of the items the hermit gave him, yes he knew how much money the hermit had in his possession. But he didn’t kill him.  Olivier claims all he was guilty of was greed. 
Our side plot - Clara has some pieces that an art dealer is interested in showing - but when he insults Olivier while meeting with Clara at the Bistro - she confronts him and he cancels the show. But - surprise - Clara meets the Suretés chief art investigator - who shows her work to another influential dealer - and Clara’s show is back on. 
In the end - Gamache arrests Olivier - who continues to claim he didn’t do it.