In May 1980, 15-year-old Oscar Drai suddenly vanishes from his boarding school in the old quarter of Barcelona. For seven days and nights no-one knows his whereabouts. His story begins in the heart of old Barcelona, when he meets Marina and her father German Blau, a portrait painter. Marina takes Oscar to a cemetery to watch a macabre ritual that occurs on the fourth Sunday of each month. At 10am precisely, a coach pulled by black horses appears. From it descends a woman dressed in black, her face shrouded, wearing gloves, holding a single rose. She walks over to a gravestone that bears no name, only the mysterious emblem of a black butterfly with open wings. When Oscar and Marina decide to follow her they begin a journey that will take them to the heights of a forgotten, post-war Barcelona, a world of aristocrats and actresses, inventors and tycoons; and a dark secret that lies waiting in the mysterious labyrinth beneath the city streets.