Borrowed from the Secret Intelligence Service cipher department to assist Briers Allerdale - a field agent returning to 1920s London with news of a dangerous anarchist plot - Miles Siward moves into a 'couples only' boarding house, posing as Allerdale’s 'wife'. Miles relishes the opportunity to allow his alter ego, Millie, to spread her wings but if Miles wants the other agent’s respect he can never betray how much he enjoys being Millie nor how attractive he finds Allerdale.

Pursuing a ruthless enemy who wants to throw Europe back into the horrors of the Great War, Briers and Miles are helped and hindered by nosy landladies, water board officials, suave gentlemen representing foreign powers and their own increasing attraction to each other.

Will they catch their quarry? Will they find love? Could they hope for both?

The clock is ticking.