It was in 1792 that twin architects and engineers Tess and Theodore Morningstarr arrived in New York with a vision for the most complex and magnificent city the world had ever seen: a symphony of towering skyscrapers, the world’s first mass public transportation system winding amongst them, and a dazzling array of buildings and parks, all of which ran on technology no one had ever seen before. Sixty-five years later, they disappeared, but not before giving the city a parting gift: the Old York Cipher. A puzzle laid into the very city they constructed, at the end of which was promised a treasure beyond all imagining. By 2017, however, the puzzle has never been solved, and the greatest mystery of the modern world is considered little more than a tourist attraction.

Tess and Theo Biedermann, twins named for the enigmatic architects, and their friend Jaime Cruz live in a Morningstarr apartment house in Manhattan—but their home is threatened when a real estate developer announces that the city has finally agreed to sell him dozens of original Morningstarr buildings. For millions of people, the sale and likely destruction of the Morningstarrs’ vision means the end of the dream of the Old York Cipher. For Tess, Theo, and Jaime, it means the loss of their home. And they decide they’re not going to just stand by while their families are evicted. If they want to save they building, they have to prove that the Old York Cipher is real. And there’s only one way to do that: They have to solve it.