The marriage between the Duke of Marlborough and 17-year-old Consuelo Vanderbilt, the American railroad heiress, was the talk of two nations when it occurred in 1895. By 1903, the Duchess had produced the requisite heir-and-a-spare, and the Duke had taken a lover, the exotic, erotic Gladys Deacon. Kate and Charles are introduced to this uncomfortable menage a trois when they come to Blenheim Palace: Kate to work on a book about King Henry II and Fair Rosamund, and Charles to follow the trail of a team of jewellery thieves.