A man dressed as a convict is found dead on a beach near the citadel. A strange and sensitive investigation starts for Vincent Pelletier as the victim is a close friend of Mr Clergeau, the prosecutor in charge of the case. In addition, Vincent is disturbed by the arrival on the island of his half-sister, Margaux. She wants to prove the innocence of their father imprisoned in Saint-Martin-de-Ré for rape and murder. Unlike Margaux, Vincent is convinced that their father is guilty and cut all relations with him. Two different plots separated by 77 years, but somehow crossed and intermingled.