Fragile, emotionally frozen, and barely educated, the heiress Elizabeth Cavendish (1654-1734) is kept in a gilded cage by a powerful but dissolute husband, the third richest man in England. All the artistic pleasures and luxuries of the Restoration are available to her but, increasingly unstable and unable to settle in any of their mansions, she searches in vain for affection and meaning in her life. Forced to accompany her husband to Jamaica following the discovery of an enormous treasure on the Caribbean seabed, she dodges pirates to return home two years later, increasingly deranged but the wealthiest widow in the land.

Attended by celebrity doctor Hans Sloane, the " mad" duchess' s slow descent into insanity gathers pace, allowing her to be tricked into a secret marriage, which sees her quickly incarcerated by an unscrupulous aristocrat in a wing of his splendid mansion as he battles for control of her colossal inheritance, launching a record-breaking, decades-long litigation. Officially declared a lunatic by Queen Anne, Elizabeth is widowed once again and banished to the outskirts of London, where she lingers in terminal dementia for a further 25 years, outliving her entire