The body of Axel Maertens, 32, sailing and kitesurfing instructor and manager of the nautical club at Malo-les-Bains, is found suffocated with a rope on a beach, presumably by accident. Morgane notices that the knot was not tied by a sailor, such as Axel, which means it was a murder. A certain Jonathan, who at first declared to have witnessed the murder, finally confesses that he committed the crime, a situation that makes everyone feel relieved, but that Morgane and Karadec find hard to believe. They join forces to continue with the investigation.