The novel opens with a young surgical nurse, Justine Brent, nursing a mill worker whose arm has been mangled by a carding machine. She soon meets John Amherst, the mill’s assistant manager who works passionately to reform the dangerous conditions at the mill and to improve the living conditions of the workers. Amherst recognizes Justine’s intelligence and sympathy, but he quickly forgets about her when he meets and falls in love with the new mill owner, Bessy Langhope.

The narrative skips ahead three years. John Amherst has learned that his now-wife Bessy has no real interest in his plan to reform the mill, although she initially appeared to be moved by the workers’ misery. In fact, her insistence on luxury, which is funded by the profit from the mills, thwarts his desire to use her controlling interest to make significant changes. The couple encounters Justine, who knew Bessy in school. When the somewhat sickly Bessy invites her to be a private nurse to herself and her stepdaughter, Justine, who is exhausted from “difficult cases,” accepts. Justine attempts to shore up John and Bessy’s increasingly troubled marriage without success. When John is abroad, Bessy has an accident while riding her horse. Paralyzed, in constant pain, and slowly dying, Bessy is attended by a physician who advances his career with the technological feat of keeping Bessy alive, ostensibly until her husband and her father arrive to say their goodbyes. When Bessy begs Justine to let her die, Justine secretly gives her a fatal dose of morphine, an act that the physician suspects.

The narrative skips ahead again to over a year later when Amherst, who has inherited the mills from Bessy, invites her family to celebrate the opening of an emergency hospital he has built in the mill town. Justine, who had stayed on after Bessy’s death as her stepdaughter’s nurse, and Amherst become reacquainted. Their shared social and intellectual interests develop into love, and they marry. The physician who had cared for Bessy and who had, earlier, asked Justine to marry him, had developed an addiction, one that had begun while he was treating Bessy. Beginning to sink into financial ruin, he blackmails Justine. Eventually, Amherst finds out that Justine killed Bessie with morphine and, horrified, rejects her.

Justine confesses her act to Bessy’s father and negotiates a deal: She will remove herself from their lives if he allows Amherst to continue his work at the mills. Bessy’s father accepts the deal, and Justine disappears for many months until Bessy’s daughter becomes ill and begs to be reunited with Justine. A family friend explains to Amherst Justine’s arrangement to protect him and convinces him that she has suffered suitable penance. Justine is reunited with Amherst when he celebrates the opening of a gymnasium for the mill workers, a project he credits Bessy with having designed. Justine, who knows that Bessy had in fact designed the gymnasium for her private estate, a project that would have drained the funds for improving the mills, keeps silent and subverts her knowledge to her husband’s perception of the facts.