The plot centers on a case of accidental bigamy. Old Lord Audley falls in love with a beautiful young woman, Lucy, who almost could have been his granddaughter. She is cute, lively and dollish, with lots of blonde curls, the ideal type for Victorian men (think of Lilian Gish). But her past is a secret. In fact, as in the course of the novel is revealed, she was already married, although husband No. 1 left her suddenly to try his luck in Australia and didn't show his face (or let her hear from him) for three full years, so she herself also set out on a new course of life.
She realizes her dream by marrying Lord Audley, and so escaping poverty, but then husband No. 1, George Talboys, suddenly returns, setting a dramatic chain of events in motion. George disappears, a crime is suspected... The part of detective is taken on by George's friend and (a bit too accidentally) nephew of Lord Audley, Roger Audley - a typical phlegmatic member of the British upper classes who is only interested in cigars and French novels. Roger Audley is the main character in the novel, but not a very interesting one because of his idleness. He is contrasted with George Talboys, the man of impulsive action, the other extreme.
Whatever way you look at it, the most interesting person in the novel is Lucy Graham/Audley. But she doesn't have the sympathy of the author. Mary Elizabeth Braddon criticizes Lucy's childish demeanor and introduces other, stronger women as positive examples, such as the daughter of Lord Audley who is a very sportive and masculine type, or the serious and motherly sister of George Talboys. But I find Lucy very strong - appearance is not everything. After all, she has taken life-changing action when she felt bogged down and does not allow anyone to subdue her.
But she was an evil one for the Victorians - they found it shocking to see how crime could invade the sacredness of the home. That reflects Victorian urbanization and uneasiness about living together with strangers in the big city. How can you be sure people are really what they say they are? Incidentally, it was this atmosphere that gave rise to the modern detective novel.
One reason I disliked the final part of the novel is because the author has Lucy packed off to a madhouse in Belgium, something she apparently approves of. A law case would be embarrassing for the Audleys, so they get silently rid of her. But the author is unable to convince me that Lucy really is insane - on the contrary. This is apparently the way how Victorian society treated women who had become a liability. The author seemingly stands on the side of 19th century society where the administrative offense of bigamy was seen as a worse crime than locking up people (mostly women) for the rest of their lives under the pretext of insanity.
Mary Elizabeth Braddon was inspired by a real case of bigamy that had been sensationally reported in the papers just a year before she published Lady Audley's Secret in 1862. There were also parallels with her own life. She was living together with a publisher, John Maxwell, who had his wife locked up for insane in Ireland. And she herself was also close to bigamy, as for many years she was this man's de facto wife, bearing him six children, and bringing up five he had from his previous marriage (she later married him after his wife died in the asylum).
Braddon's sensation novels were inspired by similar endeavors by Wilkie Collins, who around this time wrote The Woman In White. It was sensation time in British literature and Braddon remained a popular author until her death in 1915, steadily turning out one or two novels a year. After that she was forgotten, until feminism in academia in more recent years rediscovered her work. But I argue that Braddon is betting on the wrong feminist horses. She should have taken up the case of Lucy (after all, isn't Lucy's position like that of the "mad woman in the attic" in Jane Eyre?)!
Available as an Oxford World Classic and, of course, free on Gutenberg. There is also a free Librivox recording.