The novel opens with Rebecca's journey to Riverboro, to live with her two aunts, Miranda and Jane Sawyer. Until this time, she has lived on the family farm. Rebecca is the second eldest of seven children. Most of the children have fanciful names, such as Marquis and Jenny Lind, influenced by their father's artistic background (Rebecca is named after both the heroines in Ivanhoe ). The family is quite poor, due to the number of children, Mr. Randall's inability to stick to a job, and the farm being mortgaged. At the beginning of the novel, he has been dead for three years and the family is barely scraping by. Rebecca's stay with her aunt is a chance to improve her opportunities in life and to ease the strain on her family's budget, with one less mouth to feed. Despite her impoverished background, Rebecca is imaginative and charming. She often composes little poems and songs to express her feelings or to amuse her younger brothers and sisters. It is she who named their farm "Sunnybrook".
Miranda and Jane had wanted Hannah, the eldest sister, due to her pragmatic nature and household skills, but her mother needs her at home for the same reason. She sends Rebecca instead. Miranda is unimpressed by Rebecca's imagination and sallow complexion and says she's the image of her shiftless father, Lorenzo DeMedici Randall. Miranda determines to do her duty and train Rebecca to be a proper young lady, so she will not shame the Sawyer name. Jane takes on the role of Rebecca's protector and acts as a buffer between her niece and her sister. Jane teaches Rebecca to sew, cook and manage a household. Rebecca's liveliness and curiosity brighten Jane's life and refresh her spirit. Although Rebecca strives to win Miranda's approval she finds it hard to live up to Miranda's rigid standards. Rebecca is up against Miranda's view of her as "all Randall and no Sawyer".
The middle of the novel is for the most part a description of life at Riverboro and its inhabitants. Important characters are Jeremiah Cobb and his wife Sarah, who first encounter Rebecca's charm; Rebecca's schoolfellow and best friend, Emma Jane Perkins; and Adam Ladd, a young businessman who takes an interest in Rebecca's education. Adam meets Rebecca when she and Emma Jane are selling soap to help a poor family receive a lamp as a premium. Rebecca nicknames him "Mr. Aladdin".
Rebecca proves to be a good student, especially in English, and goes on to attend high school in Wareham. In the last section of the book Rebecca has become a young lady with the same high spirit and a talent for writing. She applies for a teaching place in Augusta, but her mother falls ill and Rebecca must return to care for her and the farm. While Rebecca is away from Riverboro, Miranda dies and leaves the Sawyer house and land to Rebecca. A railway company will buy Sunnybrook Farm for construction purposes and this gives the Randall family a sufficient living. Thanks to Miranda's will, Rebecca now has enough money to become an independent woman and help her siblings. The novel ends with her exclaiming, "God bless Aunt Miranda! God bless the brick house that was! God bless the brick house that is to be!"