Cat lives in Los Angeles, far away from the house in Edinburgh where she and her estranged twin sister, El, grew up. As girls, they invented Mirrorland, a dark, imaginary place full of pirates, witches, and clowns. Cat rarely thinks about their childhood home, or the fact that El now lives there with her husband. But when El mysteriously disappears, Cat is forced to return. The house is still full of shadowy corners, and at every turn Cat finds herself stumbling on long-held secrets and ghosts from the past. Because someone a�" El? a�" has left Cat clues in almost every room: a treasure hunt that leads right back to Mirrorland, where she knows the truth lies waiting.