Castle and Beckett look into the murder of a young mathematician gunned down with a 200-year-old bullet, sparking some wild theories by Castle about a time-travelling killer. When their investigation uncovers a tricked-out DeLorean and a suspect in Victorian clothing, is it possible Castle's crazy idea has some validity? This winding tale twists through the towers of Wall Street and into the world of a secret steampunk society that embraces the romance and simplicity of the past, while coupling it with the hope and promise of the future.