London architect Kate Benedict’s weekend begins with “Follow that taxi!” and takes us off and running on an international adventure, ticking off picturesque locales from London to a Venetian palace and the Tuscan countryside, pausing just long enough for a quick tour of Florence’s Duomo, with danger dogging every step.

What began as a quiet Friday night dinner date with an old childhood friend turns into a manhunt when Kate arrives at the rendezvous just in time to find her friend Ethan leaving in a taxi. Kate is alarmed to see the distinctive swirling pattern above his head that only she can see, and that she has come to recognize as a portent of imminent and mortal danger.

When Ethan’s disappearance is followed by a series of cryptic text messages, Kate goes looking for him at his sister’s house in Florence, Italy, but it’s immediately clear that whatever danger threatened Ethan in London has followed Kate to Italy—his sister Claire has a matching aura! Soon Kate and Claire are on the run, constantly watched and followed, repeatedly threatened and attacked … never knowing who they can trust. Apparently not even the carabinieri, who are now also following them.

An ancient book that was given to Claire and Ethan’s grandfather after the close of World War II holds the key to Ethan’s disappearance and the source of the danger indicated by Claire’s ever-present aura. Until all the pieces come together—the book, a hidden key, a diagram, and a cypher that Kate will need help decoding—Kate and her friends are safe; but then their tormentors—whoever they are—will have no reason to keep them alive.