Walter Hartright, a young art teacher, is walking from Hampstead to London. On this he meets a mysterious woman dressed in white, apparently in deep distress. He helps her on her way to London but later learns that she has escaped from an asylum. The next day he travels north to Limmeridge House, on a commission of being drawing master to residents of the house, previously gained by his devoted friend, an Italian language professor named Pesca. The household comprises Mr Frederick Fairlie, and Walter's students: Laura Fairlie, Mr Fairlie's 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 mentally disadvantaged Anne had lived for a time in Cumberland as a child and was devoted to Laura's mother, who first dressed her in white.
Walter and Laura quickly fall in love. Laura, however, has promised her father that she will marry Sir Percival Glyde, and Marian advises Walter to leave Limmeridge. Anne, 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 for 6 months. Hartright also leaves England, joining an expedition to Honduras. After their honeymoon, Sir Percival and Lady Glyde return then to his family estate in Hampshire, Blackwater Park. They are accompanied by Glyde's friend, Count Fosco. 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. While Marian is hearing about their plan, it is also raining and then 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 nurse and Laura escapes. Hartright has safely returned and the three live together in obscure poverty, determined to restore Laura's identity. After some time Walter discovers Glyde's secret, which was known only to Anne's mother and which Anne only presumed to know: several years earlier, Glyde had forged an entry in the marriage register at Old Welmingham Church to conceal his illegitimacy and hence unlawful inheritance of estate and title. 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. On returning to London to resume his battle with Fosco, Hartright marries Laura. When he secretly tails Fosco to investigate about him, Hartright also discovers that Fosco belongs to, and has betrayed, an Italian secret society (dubbed "The Brotherhood"), of which Pesca is a high-ranking member with enough authority to dispatch him. Using Fosco's weakness as bargaining chip, Hartright now has the power to force a written confession from Fosco and Laura's identity is restored. Fosco departs from England in haste, only to be discovered by the Brotherhood's agents some time later and murdered. Hartright and Laura have married and, on the death of Frederick Fairlie, their son becomes the Heir of Limmeridge.
