One city. Six million skeletons. And a secret that refuses to stay buried.

Twelve-year-old Mike has always preferred solving puzzles on paper to adventuring in real life. But when his family travels to Paris, he discovers a rusted brass compass in his grandfather’s luggage that doesn’t point North, it points down. Following the needle's erratic pull, Mike and his artistic older sister, Sophie, find themselves slipping through a hidden grate into the world’s largest grave: the Paris Catacombs. What starts as a hunt for a family legend quickly turns into a high-stakes race against a crew of professional thieves planning the heist of the century.

Armed only with Sophie’s sketchbook of secret maps, a mysterious "Owl Key," and their brave (but easily distracted) terrier, Bernie, Mike must navigate a subterranean maze of Ancient Riddles Solving musical ciphers etched into limestone walls. Shadowy Traps Outsmarting villains in the damp, bone-lined tunnels beneath the City of Light. Hidden History: Discovering that some legends are far more dangerous when they’re true. From the sweet-smelling kitchens of a Parisian bakery to the rushing waters of the underground sewers, Mike must find his courage before the flashlight dies. In the dark, the only way out is to solve the mystery of the Catacombs or become a permanent part of the history within them.