British spy James Locke has seen some odd events—but nothing quite as fantastical as when, in the midst of a moonlit safe-cracking mission, he witnesses a ruby necklace being spirited away as if by conjurer's trick. Following the jewels leads him to Lusinda Havershaw, who's inherited the talent of turning invisible in the moonlight—at least, the parts of her that are unclothed. To support her sisters, Lusinda slips naked through the London night to recover lost or stolen items. After enlisting her reluctant services for the Crown, Locke trains Lusinda in espionage—though her close proximity is bewitchingly distracting. And as their mission to track Russian spies grows treacherous, they'll find that the heart behaves even more mysteriously than Lusinda in the moonlight.