(from: Wilkie Collins: An Illustrated Guide © Andrew Gasson 1998, used with permission)
Walter Hartright, a young drawing-master, has secured a position in Cumberland on the recommendation of his old friend Professor Pesca, a political refugee from Italy. While walking home from Hampstead on his last evening in London, Hartright meets a mysterious woman dressed in white, apparently in deep distress. He helps her on her way but later learns that she has escaped from an asylum. The next day he travels north to Limmeridge House. The household comprises Mr Frederick Fairlie, a reclusive valetudinarian; Laura Fairlie, his niece; and Marian Halcombe, her devoted half-sister. Hartright finds that Laura bears an astonishing resemblance to the woman in white, called Anne Catherick. The simple-minded Anne had lived for a time in Cumberland as a child and was devoted to Laura's mother, who first dressed her in white.
Hartright and Laura fall in love. Laura, however, has promised her late father that she will marry Sir Percival Glyde, and Marian advises Walter to leave Limmeridge. Anne Catherick, after sending a letter to Laura warning her against Glyde, meets Hartright who is convinced that Glyde was responsible for shutting her in the asylum. Laura and Glyde marry in December 1849 and travel to Italy. Hartright also leaves England, joining an expedition to Honduras.
After their honeymoon, Sir Percival and Lady Glyde return the following June to his family estate in Hampshire, Blackwater Park. They are accompanied by Glyde's friend, Count Fosco, who married Laura's aunt, Eleanor Fairlie. Marian Halcombe is also living at Blackwater and learns that Glyde is in financial difficulties. Sir Percival unsuccessfully attempts to bully Laura into signing a document which would allow him to use her marriage settlement of £20,000. Marian now realizes that Fosco is the true villain and is plotting something more sinister, especially as Anne has reappeared, promising to reveal to Laura a secret which will ruin Glyde. Marian eavesdrops on Fosco and Glyde but is caught in the rain. She collapses with a fever which turns to typhus. While she is ill Laura is tricked into travelling to London. Her identity and that of Anne Catherick are then switched. Anne Catherick dies of a heart condition and is buried in Cumberland as Laura, while Laura is drugged and placed in the asylum as Anne Catherick. When Marian recovers and visits the asylum hoping to learn something from Anne Catherick, she finds Laura, supposedly suffering from the delusion that she is Lady Glyde.
Marian bribes the attendant and Laura escapes. Hartright has safely returned and the three live together in obscure poverty, determined to restore Laura's identity. Exposing the conspiracy depends on proving that Laura's journey to London took place after the date on the death certificate. While looking for evidence, Hartright discovers Glyde's secret. Several years earlier, Glyde had forged the marriage register at Old Welmingham Church to conceal his illegitimacy. Glyde attempts to destroy the register entry, but the church vestry catches fire and he perishes in the flames. Hartright then discovers that Anne was the illegitimate child of Laura's father, which accounts for their resemblance.
Hartright hopes that Pesca can identify Fosco but to his surprise finds that the Count is terrified when he recognizes Pesca as a fellow member of a secret society. Hartright now has the power to force a written confession from Fosco and Laura's identity is restored. Hartright and Laura have married and, on the death of Frederick Fairlie, their son becomes the Heir of Limmeridge.