An elderly couple, both religious fanatics, barricade themselves in their home and begin shooting from the windows for no apparent reason, until Montalbano's men finally succeed in disarming them.
There seems to be no justification for the couple's insane actions, but police find a strangely disfigured inflatable doll inside their home. A few days later, an identical doll is discovered in a rubbish bin and Montalbano has a bad premonition.
Soon enough, a series of strange occurrences develops - disquieting letters bearing riddles are delivered to Montalbano at Vigata police station, while a young woman is reported missing. A disturbed maniac is playing a macabre game of treasure hunt with the inspector and the riddle must be solved before it's too late.