In 1778, Dorothy Wordsworth's mother died, and the six-year-old Dorothy was sent to live with extended family. She never returned to the family home, and it was not until adolescence that Dorothy became reacquainted with her brother William. The two formed an intense and passionate emotional bond. By 1794 they were living together from that time would rarely be physically separated for more than a few weeks at a time, for the rest of their lives.

Written in the voice of Dorothy, Beloved traces the progression of their relationship, from the ecstatic infatuation of youth onwards, drawing upon Dorothy's diaries and letters as well as the recollections of friends and family members and literary and biographical scholarship.