(from: Wilkie Collins: An Illustrated Guide © Andrew Gasson 1998, used with permission)
The main story begins in 1875. Helena and Eunice are sisters brought up by their father, the Revd Abel Gracedieu. He has deliberately kept them in ignorance of their true ages because the elder daughter was adopted in 1858, after her natural mother was executed for the brutal murder of her husband. The story's main narrator is the prison governor who always feared the adoption would end badly because of the taint of inherited evil.
The household is joined by the minister's impoverished cousin, Selina Jillgall, to whom Helena takes an immediate dislike. The good-natured Eunice, however, becomes very friendly with Miss Jillgall whose only other ally is a Mrs Tegenbruggen. In 1858 she was Elizabeth Chance, lover of Eunice's murdered father. Mrs Tegenbruggen is determined to discover which daughter is the elder and make trouble.
Eunice, visiting friends in London, meets young Philip Dunboyne. They fall in love but when Philip later sees Helena she captures his affections. Eunice finds out and to compose herself takes some of her ailing father's medicine. Under its influence she sees a ghostly apparition of her executed mother urging her to kill Helena in revenge. Helena incurs the disapproval of both Gracedieu and Philip's father, brother-in-law to the executed woman. The distressed minister asks the governor for advice. Helena, to spite them both, provokes her father and, permanently unhinged, he tries to kill the governor with a razor.
Philip tires of Helena and wishes to marry the now reluctant Eunice. Helena in revenge obtains digitalis with a forged prescription and attempts to poison Philip while pretending to nurse him. She is arrested and sent to prison. Philip fully recovers and finally persuades Eunice to marry him, but only after the governor, to thwart the mischievous Mrs Tegenbruggen, has revealed the truth: it was Eunice who was adopted after her mother's execution. Helena serves a two-year sentence and emigrates to America where she prospers as the leader of a women's religious cult.