The story 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 the 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 are barely scraping by. Therefore, Rebecca's stay with her aunt is both a chance to improve her opportunities in life and to make things easier, as there is one less mouth to feed. Despite her impoverished background, Rebecca is an imaginative and charming child, often composing little poems and songs to express her feelings or to amuse her younger brothers and sisters. It is she who names their farm "Sunnybrook".
Miranda and Jane had wanted Hannah, the eldest sister, due to her pragmatic nature and household skills, but as these skills are also greatly valued by her mother, Rebecca is sent instead. Miranda is unimpressed by Rebecca's imagination and sallow complexion, saying that she is 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, acting as a buffer between her niece and her sister, and teaches Rebecca to sew, cook and be a proper little housekeeper. In return, 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 the older aunt's high standards, as she has to fight against Miranda's view of her as "all Randall and no Sawyer".
The middle part of the novel is taken up with describing the life of Riverboro and the people who live there. Important characters include Jeremiah Cobb, who is the first resident to encounter Rebecca and be charmed by her; Sarah Cobb, his wife; Rebecca's best friend, Emma Jane Perkins, and Adam Ladd, a young businessman, who first meets Rebecca when she and Emma Jane are selling soap for charity. Rebecca nicknames him "Mr Aladdin", as he gave her and Emma Jane a lamp as a present.
Rebecca proves to be a good student, especially in English, and goes on to attend the high school in Wareham. During the last part of the book, she matures into a young lady, but still retains her high spirits and develops her talent for writing. She applies for a teaching place at Augusta, but her mother falls ill and Rebecca has to return to take care of her and the farm. While Rebecca is away from Riverboro, Miranda dies, having willed the Sawyer house to Rebecca. A railway company will buy Sunnybrook Farm in order to build on the land, giving the Randall family enough to live on. Thanks to Miranda's will, Rebecca now has enough money to become an independent woman and help her brothers and sisters. 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!"