Nineteen-year-old Margaret Hale happily returns home from London to the idyllic southern village of Helstone after her cousin Edith marries Captain Lennox. She lived for almost 10 years in the city with Edith and wealthy Aunt Shaw to learn to be a young lady. Margaret has refused an offer of marriage from the captain's brother, Henry, an up-and-coming barrister. Her life is turned upside down when her father, the local pastor, leaves the Church of England and the rectory of Helstone as a matter of conscience; his intellectual honesty has made him a dissenter. At the suggestion of Mr. Bell, his old friend from Oxford, he settles with his wife and daughter in Milton-Northern (where Mr. Bell was born and owns property). The industrial town in Darkshire (a textile-producing region) manufactures cotton and is in the middle of the Industrial Revolution; masters and workers are clashing in the first organised strikes.
Margaret finds the bustling, smoky town of Milton harsh and strange, and she is upset by its poverty. Mr. Hale (in reduced financial circumstances) works as a tutor; one of his pupils is the wealthy and influential manufacturer John Thornton, master of Marlborough Mills. From the outset, Margaret and Thornton are at odds with each other; she sees him as coarse and unfeeling, and he sees her as haughty. He is attracted to her beauty and self-assurance, and she begins to admire how he has risen from poverty.
During the 18 months she spends in Milton, Margaret gradually learns to appreciate the city and its hard-working people, especially Nicholas Higgins (a union representative) and his daughter Bessy, whom she befriends. Bessy is ill with byssinosis from inhaling cotton dust, which eventually kills her. Margaret's mother is becoming sicker, and a workers' strike is brewing.
Masters and hands (workers) do not resolve the strike, and an incensed mob of workers threatens Thornton and his factory with violence after he brings Irish workers to his mill. Margaret begs Thornton to intervene and talk to the mob, but he only fuels their anger. Margaret intervenes, and is hit by a stone. Soldiers arrive, the mob disperses and Thornton carries Margaret indoors, professing his love to her unconscious figure.
Thornton proposes; Margaret declines, unprepared for his declarations of love and offended by assumptions that her action in front of the mob meant that she cared for him. Mrs. Thornton, wary of Margaret's southern (haughty) ways, is galled by Margaret's rejection of her son.
Margaret's long-absent brother, Frederick (who is wanted for naval mutiny), secretly visits their dying mother. Thornton sees Margaret and Frederick together, and assumes that he is her lover. Leonards, a man from Helstone, later recognises Frederick at the train station. They argue; Frederick pushes Leonards away, and Leonards dies shortly afterwards. When the police question Margaret about the scuffle she lies and says she was not present. As the magistrate investigating Leonards' death, Thornton knows that Margaret lied but declares the case closed to save her from possible perjury. Margaret is humbled by his deed on her behalf; she no longer only looks down on Thornton as a hard master, and begins to recognise the depth of his character.
Nicholas, at Margaret's prodding, approaches Thornton for a job and eventually obtains one. Thornton and Higgins learn to appreciate and understand each other. Mr. Hale visits his oldest friend, Mr. Bell, in Oxford. He dies there, and Margaret returns to live in London with Aunt Shaw. She visits Helstone with Mr. Bell and asks him to tell Thornton about Frederick, but Mr. Bell dies before he can do so. He leaves Margaret a legacy which includes Marlborough Mills and the Thornton house.
Thornton is forced to limit production due to market fluctuations and the strike, and fears bankruptcy. He learns the truth about Margaret's brother from Nicholas Higgins, and comes to London to settle his business affairs with Margaret. When Margaret presents Thornton with a generous business proposal, he realises that she is no longer indifferent or antagonistic to him. He again proposes marriage and, since she has learned to love him, she accepts.