Since ancient times their name has been spoken only in hushed tones. Cloaked in anonymity, they guard history's greatest and most devastating secret.

In the early twentieth century, when Barcelona was celebrated as the center of modernist art and design, the grand master of an ancient religious brotherhood prepares to die—passing the care of a sacred relic to a prominent member of his order, the revered artist and architect Antonio Gaudí. The relic, an artifact dating back to the early Christian era, could prove disastrous if it were to fall into inappropriate hands—and many secret societies, some driven by purest evil, inhabit the dark underworld that exists beneath the city's brilliant creative glow.

Nearly a century later, María, the granddaughter of the great architect's apprentice, unwittingly finds herself entrusted with a desperate mission. Following clues, with the help of her mathematician boyfriend, that are embedded in a cryptic message left by her grandfather and in the intricate symbolism of Gaudí's designs, María must race against time to unearth the fabled lost object and discover its true meaning . . . with violence, catastrophe, and death in terrifyingly close pursuit.