The wildly beautiful, deep green island of Ouessant lies at the very edge of Brittany, amidst the elemental forces of the Atlantic. It is there that Inspector Dupin, on special assignment from the prefect, is asked to solve a mysterious death: a penniless Celtic musician washes up dead on the shore shortly before the most important festival of the year. The police discover a clue in his house that is linked to an ancient dark rite, but the remote island's close-knit community makes it difficult for Dupin to investigate - sirens, priestesses and storytellers live here outside the norms and know: It's the invisible that counts. And Dupin sets himself the almost impossible task of finding out what that might be.