(from: Wilkie Collins: An Illustrated Guide © Andrew Gasson 1998, used with permission)
In 1829, Mrs Treverton lies dying in Porthgenna Tower on the west coast of Cornwall. She dictates to her maid, Sarah Leeson, a death-bed confession for her husband, Captain Treverton. Mrs Treverton dies before she can make Sarah swear to give it to the Captain but she has already extracted promises neither to destroy the paper nor to take it away from the house. Sarah therefore hides the confession in the Myrtle Room in the derelict north wing and after leaving a note for the Captain disappears from the house.
Fifteen years later, the young Rosamond Treverton is privately married by Dr Chennery to the blind Leonard Frankland. Chennery is a friend of both families and knows that Leonard's father had bought Porthgenna Tower from the Captain after the death of Mrs Treverton. The Captain had quarrelled with his misanthropic brother, Andrew Treverton, who had insulted the Captain's wife, a former actress. Andrew lives a miserly existence in Bayswater with his equally unpleasant servant, Shrowl.
A few months later, while Rosamond and Leonard are travelling through the Somerset village of West Winston on their way to Porthgenna, Rosamond is taken ill and gives birth prematurely. They engage a temporary nurse, Mrs Jazeph, who alarms Rosamond by her strange behaviour and a warning to keep out of the Myrtle Room. Mrs Jazeph is dismissed and flees to Truro to visit her uncle, Joseph Buschmann. He is delighted to see his niece, in reality Sarah Leeson, after an interval of many years. Uncle Joseph, a passionate lover of Mozart which he plays incessantly on his musical box, plans to help Sarah recover the secret paper from Porthgenna. They gain entrance to the house but before Sarah can reach the letter she faints, terrified by mistaking the wind for a ghost. Sarah leaves Porthgenna and to avoid any further questions loses herself in the anonymity of London.
Dr Chennery writes to Andrew Treverton on Rosamond's behalf to identify the Myrtle Room. Treverton refuses to help but Shrowl sells a copy of the plans and the Dead Secret is revealed. Rosamond is really the illegitimate daughter of Sarah Leeson whose lover, Hugh Polwheal, was killed in a mining accident before they could marry. The childless Mrs Treverton passed off Rosamond as her own, both to preserve the love of her husband and to save Sarah's reputation. Rosamond's inheritance of £40,000, the purchase price of Porthgenna paid by Leonard's father, rightfully belongs to the dead Captain's brother. Andrew Treverton, staggered at finding two people who don't care about money, forces them to take it back. Uncle Joseph has meanwhile traced Sarah so that mother and daughter are reunited. The frail Sarah dies happy in Rosamond's arms, at peace because the ghost which perpetually haunted her has finally disappeared. She is buried in Hugh Polwheal's grave in Porthgenna.