It is a truth universally acknowledged that the life of a former spy cannot remain quiet for long.
London, December 1820.
Former spies Malcolm and Mélanie Rannoch are hosting a small party and looking forward to an intrigue-free holiday season when their friend, Thomas Thornsby, turns up dripping wet on the doorstep of their Berkeley Square house.
Not only is Thomas clearly overwrought, his arrival is particularly awkward. Thomas is newly betrothed to an heiress. Edith Simmons, the woman he wished to marry - if not for lack of fortune - is one of the Rannochs' guests that evening. But Thomas says he had no one else to turn to. His fiancée, the decorous Marianne Schofield, has vanished without a trace from a Mayfair holiday ball.
Always willing to help a friend, the Rannochs don evening attire and join the elegant gathering. Amidst the holiday revelry they find that Thomas's seemingly perfect betrothed has secrets of her own. As they uncover layers of mystery, Mélanie and Malcolm realize some of the plots being hatched beneath the mistletoe may prove deadly.