Four masked horsemen, dressed as Templar Knights, make their way from Central Park to the Metropolitan Museum of Art. They do not stop at the steps, but steer the horses through the crowds gathered for opening of the museum's exhibition of Vatican treasures, scattering the great and the good of Manhattan society and storming through to the exhibition, collecting artefacts, opening gunfire on security and police, beheading a security guard with a broadsword. Among the melee, archaeologist Tess Chaykin observes the leader of the horsemen head for one piece in particular, observing the reverence with which he selects the object and utters words in Latin as he makes away with it. The four horsemen leave the stunned crowd and disappear into the urban nightscape of Manhattan. The FBI's investigation team is led by Sean Reilly, a practising Catholic, and involvement from the Vatican in the guise of De Angelo, a senior priest. When it is discovered that the piece stolen by the leader of the horsemen is an ancient decoding device used by the Knights Templar, Tess Chaykin becomes more than a witness to the event, and is drawn into the investigation by her professional knowledge which leads her to aid them in pursuing a former colleague, Vance, whose obsession is fuelled by the most personal of tragedies. She leads the team, and Reilly in particular, through the dark history of the crusading Knights and on a journey which takes herself and Reilly through the sewers of New York, over continents to the barren Turkish coastline, and from there to the highest point of command at the Vatican, culminating with a dramatic sea chase amid a fierce storm. As Tess is compelled by curiosity and a critical desire for archaeological truth, so her growing relationship with Reilly plunges him into spiritual and professional conflict, the legacy of which is the secret for which the original Knights Templar were persecuted and tortured. Vance's desperate wish for its revelation threatens to shake all modern religions to the very core, leaving Reilly and Tess with the burden of this truth.