Frances "Fanny" Price, at age ten, is sent from her family home to live with her uncle and aunt in the country in Northamptonshire. It is a jolting change, from the elder sister of many, to the youngest at the estate of Sir Thomas Bertram, husband of her mother's older sister. Her cousin Edmund finds her alone one day and helps her. She wants to write to her older brother William. Edmund provides the writing materials, the first kindness to her in this new family. Her cousins are Tom Jr. (age 17), Edmund (16), Maria (13) and Julia (12). Her aunt, Lady Bertram, is kind to her, but her uncle frightens her (unintentionally) with his authoritative demeanour. Fanny's mother has another sister, Mrs Norris; the wife of the clergyman at the Mansfield parsonage. Mrs Norris and her husband have no children of their own, and she takes a 'great interest' in her nieces and nephews; Mrs Norris makes a strict distinction between her Bertram nieces and lowly Fanny. Sir Thomas helps the sons of the Price family find occupations when they are old enough. William joins the Navy as a midshipman not long after Fanny arrives at Mansfield Park. He visits them once after going to sea, and writes to his sister.
When Fanny is fifteen, Aunt Norris is a widow and moves into a small cottage. The frequency of her visits to Mansfield Park increases, as does her mistreatment of Fanny. Tom Bertram incurs a large debt and to pay it, Sir Thomas sells the living of the parsonage, freed up by the death of Uncle Norris, to clergyman Dr Grant.
When Fanny is sixteen, Sir Thomas leaves to deal with problems on his plantation in Antigua. He takes Tom along and trusts to Aunt Norris for the others. Mrs Norris takes on the task of finding a husband for Maria and finds James Rushworth, with income of ₤12,000 a year, but weak-willed and stupid. Maria accepts his marriage proposal, subject to Sir Thomas's approval on his return. After a year in Antigua, Sir Thomas sends Tom home to Mansfield Park.
One year later, the wealthy and worldly Henry Crawford and his sister, Mary Crawford, arrive at the parsonage to stay with Mrs Grant, their half-sister. The arrival of the fashionable Crawfords enlivens life in Mansfield and sparks romantic entanglements. Mary and Edmund begin to form an attachment. She is disappointed to learn that Edmund will be a clergyman, due to her love of fashionable society. Fanny fears that Mary's charms and attractions have blinded Edmund to her flaws in morality. On a visit to Mr Rushworth's estate Sotherton in Henry Crawford's barouche, Henry deliberately plays with the affections of both Maria and Julia. Maria believes Henry is falling in love with her and treats Mr Rushworth dismissively, provoking his jealousy, while Julia struggles with jealousy and resentment towards her sister. Fanny observes this while Aunt Norris, blinded by her self-importance and Edmund, infatuated with Mary, fail to perceive the various flirtations.
Encouraged by Tom and his friend Mr Yates, the young people decide to put on an amateur performance of the play Lovers' Vows after their return to Mansfield. Edmund objects, believing Sir Thomas would disapprove and feeling that the subject matter of the play is inappropriate for his sisters. Edmund reluctantly agrees to take on the role of Anhalt, the lover of the character played by Mary Crawford, to prevent an outsider from playing the part. The play provides a pretext for Henry and Maria to flirt in public. Fanny observes this, but Aunt Norris, caught up in the excitement of staging a play, does not.
Sir Thomas arrives home earlier than expected, while most are in the midst of rehearsal. He stops the play. Henry, from whom Maria had expected a marriage proposal, instead takes his leave, and she is not pleased. She goes ahead with marriage to Rushworth in order to prove she is unaffected, with her father's permission. They honeymoon in Brighton and then settle in London, taking Julia with them. Fanny's improved appearance and gentle disposition endear her to Sir Thomas. With Maria and Julia gone, Fanny and Mary Crawford visit often.
Henry returns to Mansfield parsonage, deciding to entertain himself by making Fanny fall in love with him. Fanny's brother William visits Mansfield Park, much to the delight of Fanny. Her familial love and delicate manner make Henry fall in love with Fanny, regardless of her lack of response to his attentions. Sir Thomas, noticing Henry's attentions to Fanny, approves the match and holds a ball to celebrate Fanny's coming out, an important distinction in the Regency era. Fanny borrows a necklace from Mary to hold a cross she has received as a gift from William, but is distressed to learn the chain was a gift to Mary from Henry. Immediately following, she receives a simpler chain that suits her much better from Edmund, and the cross fits on this chain. She wears both to the ball. Edmund, meanwhile, asks Mary for the first two dances at the ball, which she accepts while attacking his career choice, deterring his plan to propose and souring his mood for the ball. Fanny receives the honor of leading the dance, which in her modesty is a surprise to her. William leaves early the next day in Henry's barouche, with a stop in London before returning to Portsmouth. Edmund follows a few days later to take orders for the clergy, upsetting Mary.
Henry returns, announcing to Mary his intention to marry Fanny, a decided turn from his original notion. To further his suit, he uses his family connections to help Fanny's brother William gain promotion as a naval lieutenant, to her great joy and gratitude. When Henry proposes marriage, however, Fanny rejects him out of hand, due to his moral failings, observed during the play rehearsals. Sir Thomas is astonished at her refusal; she does not tell him about Henry's behavior during the play, afraid of incriminating Maria. He reproaches her, accusing her of ingratitude, and encourages Henry to persevere. Edmund returns from his absence, which he had prolonged, hoping to avoid meeting Mary. He quickly begins to fall for Mary again.
To help Fanny appreciate her life in a wealthy house, Sir Thomas sends her for a visit to her parents in Portsmouth. She sets off with William and sees him in his first berth as a commissioned officer. At Portsmouth, she develops a firm bond with her younger sister, Susan, but is taken aback by the contrast between her surroundings—noise, chaos, unpalatable food, crude conversation, and filth everywhere—and the harmonious environment at Mansfield. Henry visits her there. Although Fanny still refuses him, she sees some of his good features, in dealing with her family and managing his own estate.
Henry leaves for London, and shortly afterward, Fanny learns that scandal has enveloped him and Maria. The two meet at a party and rekindle their flirtation, which leads to an affair. An indiscreet servant makes the affair public and the story is in the newspapers. Maria runs away with Henry. Mr Rushworth sues Maria for divorce, and the proud Bertram family is devastated. At the same time, Tom has fallen gravely ill as a result of his irresponsible lifestyle, and Julia, fearing her father's anger for her part in concealing Maria's affair, has eloped with Tom's friend, Mr Yates.
Edmund takes Fanny back to Mansfield Park along with Susan. A repentant Sir Thomas now realises that Fanny was right to reject Henry's proposal, and now regards her as his own daughter. During an emotional meeting with Mary Crawford, Edmund discovers that Mary does not condemn Henry and Maria's adultery, and regrets only that it was discovered. She places blame on Fanny for failing to accept Henry right away. Edmund is devastated; he breaks off the relationship and returns to Mansfield Park.
Edmund slowly gets over his love for Mary. Then he comes to realise how important Fanny is to him. He declares his love for her, and they are married, live at Thornton Lacey, and eventually move to Mansfield parsonage, in the circle of those they love best. Susan takes Fanny's place as the companion of Lady Bertram. Tom recovers from his illness, a steadier and better man for it, and Julia's husband, Mr Yates, proves to be a respectable member of the family. Henry Crawford refuses to marry Maria. Her family will not take her in, but her father sets her up in a house with Aunt Norris, the both of them out of his sight. Mary Crawford moves in with Mrs Grant, hoping for a husband.