When 12 year-old Mia Cornell is sent to live at the Tarpley Estate, her ninth foster home, she has no idea what she's in for. Having a kind but mysterious foster mother and three quirky foster sisters is all well and good. Discovering that her new family is a secret society—they use a portal in their garden labyrinth to travel the globe seeking lost art, artifacts, and OSOs (objects of supernatural origin)—is something else entirely.

Mia barely has time to wrap her head around the truth when the Society's latest job, tracking down Marie Antoinette's necklace, goes sideways. Mia and her new siblings—Tomi, the historian; Devon, the hacker; and Lily, the muscle—must use their wits, plus a few cool OSOs filched from the Society's vault, to stay a step ahead of a sinister enemy from their foster mother's past. But just how far back does Mrs. Tarpley's past go? And will Mia stand by her new family or cut and run when they need her the most?