A masterful blend of Gothic drama and romance, Wilkie Collins's mystery novel is an exploration of illegitimacy and inheritance. Set in Cornwall, the plot foreshadows The Woman in White with its themes of doubtful identity and deception, and involves a broad array of characters. The 'secret' of the book's title is the true parentage of the book's heroine, Rosamond Treverton, which has been written down and kept in an unused room at Porthgenna Tower. This is where, 20 years later, much of the novel's action is set.