(from: Wilkie Collins: An Illustrated Guide © Andrew Gasson 1998, used with permission)
Valentine Blyth, an artist, and Lavinia, his invalid wife, have an adopted deaf and dumb daughter, Mary, usually called Madonna because of her beautiful eyes and resemblance to a painting by Raphael. There is a mystery about her origins. She was rescued by Blyth from a travelling circus, where she was mistreated by the cruel proprietor after she lost her hearing in an accident. Years before, her dying mother had been helped by the wife of a circus clown, the good-natured Mrs Peckover, who brought up the orphaned baby as her own. Mrs Peckover agrees to Mary's informal adoption by the Blyths since they can offer her a better life. The only clue to her original identity is a hair-bracelet with the initials MG which Blyth keeps locked in his bureau for fear that Mary's unknown family might one day claim her.
Zachary Thorpe (Zack), closely attached to his gentle mother but at odds with his over-bearing, religious father, is a frequent visitor to the Blyths despite Mr Thorpe's disapproval of Madonna's dubious origins. Madonna falls in love with him, but Zack is too immature to respond. He is a wild 20-year-old who secretly frequents late-night theatres and drinking places against his father's wishes. One night he is involved in a brawl and is rescued by Mat Marksman, an outlandish, nomadic character who was scalped by Indians. Zack arrives home drunk and the following day leaves his father's house to share lodgings with his new-found friend.
Mat, in search of his lost sister, visits his aunt, old Joanna Grice, in the Midlands town of Dibbledean and hears the family scandal which occurred after he went abroad. His sister Mary left home, pregnant with an illegitimate child, and is buried in a pauper's grave in Bangbury. Mat takes a box containing Mary's old love letters as well as a note from Joanna Grice justifying her conduct. The letters identify Mary's lover as Arthur Carr. Carr had written several times but Joanna intercepted his letters. Thinking herself deserted, Mary left home to avoid disgracing the family without realizing that her father had forgiven her.
On his return to London, Mat accompanies Zack to Blyth's studio. He is struck by the resemblance between Madonna and his dead sister and from a comment by Zack guesses that the clue to her origin is locked in Blyth's bureau. Mat gets Blyth drunk, takes an impression of his key and obtains the bracelet. He is resolved to find Arthur Carr and take revenge.
Mat, recognizing that the distinctive brown hair in the bracelet is identical with Zack's, deduces that Zack's father was in reality 'Arthur Carr' and confronts him. Thorpe confesses in a letter to Blyth, while the magnanimous Mat destroys a similar note to Zack and takes him travelling in America. When news comes that Zack's father has died, Mat reveals the whole secret, including the fact that Zack and Madonna are brother and sister. Zack returns home and eventually persuades Mat to leave his solitary life to rejoin his niece and friends in England.