(from: Wilkie Collins: An Illustrated Guide © Andrew Gasson 1998, used with permission)
Madame Pratolungo, the narrator, is a Frenchwoman, widow of a South American republican. She takes the position of companion to Lucilla Finch who is now 21 and has been blind with cataracts since she was a year old. Lucilla lives in a wing of her father's house in the Sussex village of Dimchurch, near Lewes. Finch, a pompous clergyman, fell out with his first wife's family, the Batchfords. However, he has fourteen children from his second marriage and, perpetually short of money, is delighted to accept an over-generous allowance from Lucilla's personal inheritance.
Lucilla falls in love with their close neighbour, the reclusive Oscar Dubourg, whose main interest is making objects from precious metals in his work-room. Oscar is totally devoted to his identical twin brother, Nugent, who narrowly saved him from hanging by discovering crucial evidence proving him innocent of a murder.
Oscar is savagely attacked and robbed of his gold and silver plate, and sends a message for help written in blood on the frock of Lucilla's wandering three-year-old half-sister, Selina (called Jicks). Oscar appears to recover from a blow to the head but begins to suffer from increasingly bad epilepsy. He is offered a cure: prolonged treatment with silver nitrate which has the side-effect of permanently staining the skin dark blue, almost black. The marriage is postponed and, against the advice of Madame Pratolungo, Oscar misleads Lucilla into thinking that it is Nugent who has been undergoing the treatment.
Nugent, identical to Oscar except for his skin colour, has returned from America where he squandered his half of the family fortune. The only way Lucilla can tell the brothers apart is by her sense of touch which produces a 'tingle' with Oscar. Knowing of Lucilla's blindness, Nugent brings with him the eccentric and exuberant Herr Grosse, a noted German oculist who examines Lucilla in collaboration with the staid English doctor Mr Sebright. Their opinions differ but Lucilla, eager to take any chance of actually seeing her beloved Oscar, follows the advice of Grosse and has an operation.
Nugent is also in love with Lucilla and knowing her peculiar prejudice against dark colours is happy to perpetuate the confusion of identity. When the bandages are removed he contrives to be the first person seen by Lucilla and as intended is mistaken for his brother. The real Oscar, an object of pity and horror, leaves Dimchurch to go abroad, prepared to sacrifice his own happiness for that of Lucilla and the brother who saved his life. Grosse reluctantly consents to the deception because the shock of the truth may hinder his patient's recovery. He recommends Lucilla to take the air at Ramsgate and in the absence of Madame Pratolungo, in France to nurse her incorrigibly gallant old father, Lucilla is accompanied by her aunt, Miss Batchford.
Nugent writes to Lucilla pretending to be Oscar and then impersonates him at Ramsgate. He tries to press Lucilla into an immediate marriage before either Finch or Madame Pratolungo can uncover the deception. Lucilla's old sense of touch tells her that something is wrong and the stress causes her sight to deteriorate. Nugent, in desperation, tries to convince her that Madame Pratolungo is her enemy. He tricks Lucilla into leaving for London where she stays with his married relative while a marriage licence is obtained in the name of Oscar.
Madame Pratolungo has traced Oscar and they return to England to prevent the fraudulent marriage. Lucilla has once more lost her sight and immediately recognizes her true love by touch. Nugent repents and gives Oscar the marriage licence permitting the wedding two days later. Nugent, asking them to name their future son after him, joins an unsuccessful Arctic expedition and is eventually found frozen to death clutching a lock of Lucilla's hair.