When the naked body of a teenage girl is found on the banks of the River Baztán, it is quickly linked to a similar murder one month before. Soon, rumours are flying in the nearby village of Elizondo. Is this the work of a ritualistic killer or is it the basajaun, the ‘invisible guardian’ of Basque mythology? Inspector Amaia Salazar leads the investigation, taking her back to the heart of the Basque country where she was born, and where she hoped never to return. Shrouded in mist and surrounded by impenetrable forests, it is a place of unresolved conflicts and a terrible secret from Amaia’s childhood that will come back to haunt her. Faced with the superstitions of the village, Amaia must fight the demons of her past to confront the reality of a serial killer on the loose. But as she is drawn deeper into the investigation, she feels the presence of something darker lurking in the shadows…

The discovery of the tomb of William Tell’s son in a town in the Basque Country spurs the village's cantankerous citizens to lobby for Swiss annexation.