(from: Wilkie Collins: An Illustrated Guide © Andrew Gasson 1998, used with permission)
Basil is the younger son of a proud, stern father and comes of an ancient, noble family. He has a devoted younger sister, Clara, and a wild but good-natured older brother, Ralph. Travelling home, on impulse by omnibus, Basil falls in love at first sight with Margaret Sherwin, a linen-draper's daughter. He follows her home to the newly built suburbs north of Regent's Park and, after contriving a meeting, asks her father for permission to marry; but because of his own father's certain opposition, the marriage should be kept secret. Mr Sherwin agrees on condition that the marriage takes place within the week but is not consummated for one year, since Margaret is only just seventeen. The delay will give Basil time to persuade his father to accept the marriage, and he cannot be forced to withdraw from it. The marriage duly takes place and Basil spends the next few months visiting Margaret every evening under the supervision of the mildly deranged Mrs Sherwin. He tries unsuccessfully to improve Margaret's mind and after overhearing two of her tantrums begins to doubt her character.
Sherwin's confidential clerk, Robert Mannion, returns from a business trip to France. Mannion's previous background is cloaked in secrecy and he has a strange power over the family. Nevertheless, he professes friendship and uses his influence for Basil's benefit. After a strained visit to his father's country house, Basil returns to find both Margaret and Mannion changed. On the evening before his year's 'probation' is completed, Basil is disconcerted that Margaret has gone to a party and will be escorted home by Mannion. He decides to collect her himself but sees her leave early with his rival. Basil follows them to a hotel and through a thin partition wall hears Mannion seduce Margaret. Basil waits and attacks Mannion, hurling him to the ground with such force that he is permanently disfigured and loses the sight of one eye. Basil collapses into delirium.
On recovering, he realizes that Margaret is as much to blame as Mannion, despite threatening letters from Sherwin defending her. Basil confesses the ignoble marriage to his father who disowns him, tearing his name from the record in the family Bible. Mannion writes from hospital, revealing his secret past. When his father, a gentleman who lived beyond his means, was to be hanged for forgery, his patron, Basil's father, refused to intervene. Mannion had lived a miserable existence under assumed names until a friend arranged employment with Sherwin where he made himself indispensable. He had watched Margaret develop and despite her deceitful nature regarded her as his prize. Basil's marriage had compounded the family offence and Mannion resolved to take revenge by ruining his happiness and reputation.
Ralph returns from the Continent and visits Sherwin to buy his silence. He fortuitously obtains a letter written by Mannion which confirms Margaret's guilt. Margaret, visiting Mannion in hospital, contracts typhus from which she dies. Basil sees her when she is at the point of death, forgives her and leaves London for Cornwall. Mannion forces him to leave the fishing village where he is staying and confronts him on the cliff tops. While gloating over his revenge, Mannion falls to his death on the rocks below. The shock causes Basil to collapse. He is brought back to London by Ralph and Clara and reconciled with his father. After writing his history, Basil retires to the country to live quietly with Clara.