A murder case rocks a small town when Julien Monnier is found dead, stabbed near the cave dwellings known as "Les grottes du Régulus." An old book is found beside him. Grace, a police captain, is put in charge of the investigation and discovers similarities with a ten-year-old case: a 17-year-old girl, Sarah, had been found dead, also stabbed, in the same caves. Grâce calls on Alex, her childhood friend who has become an expert in old books. He had investigated Sarah's death when he was a police officer. Alex reluctantly revisits the past and agrees to help, partly to reconnect with Grâce, his lost love. Their reunion rekindles old feelings and painful memories linked to their separation. As the investigation progresses, the memories of the protagonists and suspects become intertwined. Grace and Alex relive their past, but are confronted with conflicting memories and unspoken words.