The story opens on a modest New England home one Christmas during the Civil War. The four March sisters, Meg, Jo, Beth, and Amy, grumble at being poor and fatherless, since Mr. March is away serving in the army, but rally around their mother, whom they call Marmee. On Christmas morning they give their own breakfast to a starving immigrant family, setting the book's tone: the Marches have little money but a great deal of love, and they are happiest when they share it.
Each sister carries a distinct nature and a fault to mend. Meg, the eldest and prettiest, longs for finery and ease. Jo is tall, bookish, hot-tempered, and fiercely independent, determined to write something splendid and to keep her freedom. Gentle Beth is shy and homeloving, content with her piano and her family. Amy, the youngest, is vain and artistic, set on becoming a fine lady. Much of the book follows their small domestic trials and the moral lessons each one slowly learns.
The household widens to include the boy next door, Laurie, the lonely grandson of rich old Mr. Laurence, who becomes almost a brother to the girls. The first half builds through everyday episodes, schemes, quarrels, plays, and ambitions, toward two shocks: news that the father is gravely ill, which sends Marmee rushing south while Jo sells her long hair to help pay for the journey, and Beth's near-fatal scarlet fever, caught from nursing the poor family they had befriended.
The second half carries the sisters into adulthood. Meg marries the tutor John Brooke and learns the give-and-take of a poor young household. Amy travels to Europe to study art. Laurie, grown to a man, asks Jo to marry him, and she refuses, sure she does not love him that way and unwilling to surrender her liberty. Jo goes to New York, where she writes for the papers and meets a poor, kind German scholar, Professor Bhaer.
Sorrow returns when Beth, never fully recovered, quietly fades and dies, the first of the sisters to go and, the book suggests, the one most ready. Out of that grief the others find their lasting places: Amy and Laurie marry abroad, Jo accepts Professor Bhaer and turns the inherited house, Plumfield, into a school for boys, and the surviving family gathers at the close around Marmee, counting their blessings and missing the sister who is gone.