A murder in Paris brings Commissaire Adamsberg out of the Icelandic mists of his previous investigation (book eight, A Climate of Fear) and unexpectedly into the region of Nîmes, where three old men have died of spider bites. A pensioner tells Adamsburg that two of the three octogenarians have known each other since childhood, when they lived in an orphanage called The Mercy, forming a group of violent boys known as the "band of recluses." Now Adamsberg faces two obstacles: the third man killed was not part of the club, and the amount of spider venom necessary to kill doesn't add up. When more members of the old band succumb to recluse bites, the commissaire must uncover the secret hidden behind the walls of the orphanage.