In the sealed basement of an old inherited house, Ethan Blackwell discovers something that should not exist: a window built into a wall that has no outside. Through the glass, he sees a street lined with buildings that vanished from the city a hundred years ago, moving with people who don't know they're already gone. When Mara Whitlock, a historian obsessed with the lost district, and Daniel Cross, a detective chasing a decades-old disappearance, are pulled into what the window shows, the three of them realize the glass isn't just looking backward. Something on the other side has started looking back.