It's lady's maid Louisa Cannon's wedding day, but the fantasy is shattered shortly after when she is approached by a secretive man asking her to spy on Diana Mitford—who is having an affair with the infamous Oswald Mosley—and her sister Unity. Thus as summer 1933 dawns, Louisa finds herself accompanying the Mitfords on a glitzy cruise, full of the starriest members of Society. But the waters run red when a man is found attacked. Back in London, the case is taken by lawyer Tom Mitford, and Louisa finds herself caught between worlds: of a love lost, a family divided, and a country caught in conflict.