Fanny Price, at age ten, is sent from her impoverished home in Portsmouth to live as one of the family at Mansfield Park, the Northamptonshire country estate of her uncle, Sir Thomas Bertram. There she is mistreated by all but her elder cousin Edmund. Her aunt Norris, the wife of the clergyman at the Mansfield parsonage, makes herself particularly unpleasant.
When Fanny is fifteen, Aunt Norris is widowed and the frequency of her visits to Mansfield Park increases, as does her mistreatment of Fanny. A year later, Sir Thomas leaves to deal with problems on his plantation in Antigua, taking his spendthrift eldest son Tom. Mrs Norris, looking for a husband for Maria, finds Mr Rushworth, who is rich but weak-willed and considered stupid, and Maria accepts his proposal.
The following year, Henry Crawford and his sister, Mary, arrive at the parsonage to stay with their half-sister, the wife of the new incumbent, Dr Grant. With their fashionable London ways, they enliven life in Mansfield. Edmund and Mary then start to show interest in one another.
On a visit to Mr Rushworth's estate, Henry flirts with both Maria and Julia. Maria believes Henry is in love with her and so treats Mr Rushworth dismissively, provoking his jealousy, while Julia struggles with jealousy and resentment towards her sister. Mary is disappointed to learn that Edmund will be a clergyman and tries to undermine his vocation. Fanny fears that Mary's charms are blinding Edmund to her flaws.
After Tom returns, he encourages the young people to begin rehearsals for an amateur performance of the play Lovers' Vows. Edmund objects, believing Sir Thomas would disapprove and feeling that the subject matter of the play is inappropriate for his sisters. But after much pressure, he agrees to take on the role of the lover of the character played by Mary. The play provides further opportunity for Henry and Maria to flirt. When Sir Thomas arrives home unexpectedly, the play is still in rehearsal and is cancelled. Henry departs without explanation and Maria goes ahead with marriage to Mr Rushworth. They then settle in London, taking Julia with them. Sir Thomas sees many improvements in Fanny and Mary Crawford initiates a closer relationship with her.
When Henry returns, he decides to entertain himself by making Fanny fall in love with him. Fanny's brother William visits Mansfield Park, and Sir Thomas holds what is effectively a coming-out ball for her. Although Mary dances with Edmund, she tells him it will be the last time as she will never dance with a clergyman. Edmund drops his plan to propose and leaves the next day. So too do Henry and William.
When Henry next returns, he announces to Mary his intention to marry Fanny. To assist his plan, he uses his family connections to help William achieve promotion. However, when Henry proposes marriage, Fanny rejects him, disapproving of his past treatment of women. Sir Thomas is astonished by her continuing refusal, but she does not explain, afraid of incriminating Maria.
To help Fanny appreciate Henry's offer, Sir Thomas sends her to visit her parents in Portsmouth, where she is taken aback by the contrast between their chaotic household and the harmonious environment at Mansfield. Henry visits, but although she still refuses him, she begins to appreciate his good features.
Later, Fanny learns that Henry and Maria have had an affair that is reported in the newspapers. Mr Rushworth sues Maria for divorce, and the Bertram family is devastated. Tom meanwhile falls gravely ill as a result of a fall from his horse. Edmund takes Fanny back to Mansfield Park, where she is a healing influence. Sir Thomas realises that Fanny was right to reject Henry's proposal and now regards her as a daughter.
During a meeting with Mary Crawford, Edmund discovers that Mary only regrets that Henry's adultery was discovered. Devastated, he breaks off the relationship and returns to Mansfield Park, where he confides in Fanny. Eventually the two marry and move to Mansfield parsonage. Meanwhile, those left at Mansfield Park have learned from their mistakes and life becomes pleasanter there.