In May 1980, 15-year-old Oscar Drei suddenly vanishes from his boarding school in the old quarter of Barcelona. For seven days and nights no one knows his whereabouts. It all began the previous autumn when, while exploring the dilapidated grounds of what seemed to be an abandoned house filled with portraits, he inadvertently stole a gold pocket watch.
Thus begins Oscar's friendship with Marina and her father Herman Blau, a portrait painter. Marina takes Oscar to the gardens of the nearby cemetery to watch a macabre ritual that occurs on the fourth Sunday of each month. At 10 a.m., a coach drives up to the cemetery and a woman with her face shrouded, wearing gloves, and holding a single rose is helped down from the coach and walks over to a nameless gravestone, where she sets down the flower, pauses for a moment, and then returns to the coach.
The gravestone bears no marking but the outline of a strange-looking butterfly with open wings. On one of their subsequent walks Oscar and Marina spot the same woman and determine to follow her. Thereupon begins their journey into the woman's past, and that of the object of her devotion.
It is a journey that takes them to the heights of a forgotten, postwar-Barcelona society, of now aged or departed aristocrats and actresses, inventors and tycoons; and into the depths of the city's mysterious underground of labyrinthine sewers, corrupt policemen, beggars' hovels, and criminal depravity.