The novel opens on a spring morning in a Russian town, where the narrator contrasts the gladness of the natural world with the way grown men keep cheating and tormenting one another. Three prisoners are taken from jail to court. One of them, Katerina Maslova, is a prostitute accused of poisoning a merchant and stealing his money.
On the jury that day sits Prince Dmitri Nekhludoff, a wealthy, idle nobleman. He slowly realizes that the accused woman is Katusha, a half-servant, half-ward in his aunts' house whom he seduced and abandoned years earlier when he was a young officer. His careless act, and the hundred roubles he threw at her, set off the chain of events that led her into prostitution. Through a clerical blunder the jury's verdict is botched, and although her guilt is doubtful, Maslova is sentenced to penal servitude in Siberia.
Shaken, Nekhludoff resolves to repair the harm he caused. He visits Maslova in prison, tells her he wants to marry her to atone for his sin, and is met not with gratitude but with anger and scorn. He begins to dismantle his old life. He gives the peasants the land he inherited, breaks off a comfortable engagement, and pushes through the courts and ministries to have her case reviewed, learning along the way how arbitrary and cruel the whole legal apparatus is.
When the appeals fail, Nekhludoff follows the convict convoy east. The middle and later parts of the book widen into a long indictment of Russian life: the prisons packed with people jailed for trifles, the officials who sign away lives without feeling, the state church whose service Tolstoy describes as hollow ritual, and the property laws that keep peasants poor. Among the political prisoners marching to Siberia, Maslova finds decent company for the first time and begins to recover her dignity.
Maslova's sentence is finally commuted to settlement rather than hard labour, but by then she has made her own choice. She decides to join the gentle revolutionary Simonson, telling Nekhludoff to forgive her and live his own life, knowing that marrying him would only spoil it. Left alone, Nekhludoff opens a Testament given to him by an English visitor and reads the Sermon on the Mount as if for the first time. He concludes that the business of his life is to live by those commandments, and a wholly new life begins for him, not in new circumstances, but in the new meaning he gives to everything he does.