Olivia Mercer, a map authenticator in Boston, discovers a 17th-century map depicting a city that never existed. As she investigates, she uncovers a secret society of cartographers who believed they were mapping "thin places"—locations where the boundary between worlds was permeable. But when a series of gruesome murders suggests that someone is using the maps to open these doorways, Olivia realizes she's stumbled onto a conspiracy far more dangerous than she ever imagined.

With the help of a historian, a detective, and a tech expert, Olivia must race against time to find and destroy the remaining maps before they can be used to unleash a catastrophic force upon the world. But as she delves deeper into the mystery, she discovers that her own past is inextricably linked to the maps and the secret society that created them.