Captain Jérémy Laubier investigates a young surfer's murder in La Torche, Breton, discovering a course on Ys myth. Professor Morgane Le Dantec identifies the victim's sister.

Back in Brittany, Captain Morgane Ledantec is confronted with the discovery of human bones after the fall of a sacred oak during a storm. The victim was a follower of druidism which Morgane was introduced to as a teenager by her godmother.