When Lord Muchison catches sight of a mysterious and beautiful lady in a carriage on London's Bond Street, he is captivated and spends the next few days on the lookout for her again. A week later, by chance he finds himself at a dinner party where Lady Alroy is present, who he immediately recognizes as the lady from Bond Street. But her mysterious behaviour and insistence on secrecy puzzles him greatly.

Their relationship develops via furtive notes and hastily arranged meetings. But as Lord Murchison prepares to propose marriage, a chance sighting of Lady Alroy entering a run-down house on the shabby Cumnor Street in a less salubrious area of London sets him on the trail of her mysterious secret....