At the age of ten, Fanny Price leaves her poor and overcrowded home in Portsmouth to live with her affluent uncle and aunt, Sir Thomas and Lady Bertram, at Mansfield Park in Northamptonshire. Over-awed and sometimes humiliated by her adult relatives (particularly by her snobbish and unkind Aunt Norris) and four older and more sophisticated cousins, she is initially frightened, miserable and homesick. However, with the kindness and support of her cousin Edmund, she is eventually able to find her niche in the household.
A few years later, Henry and Mary Crawford, a wealthy brother and sister, move into the neighbourhood and become friendly with Fanny's cousins at Mansfield Park. While Sir Thomas Bertram is away dealing with business matters in his West Indies plantations, the young people decide to organise some amateur theatricals and to put on a rather risqué play. This provides an opportunity for Maria (the eldest Miss Bertram) and Henry Crawford to flirt with each other, even though Maria is engaged to the dull but wealthy Mr Rushworth. Julia Bertram, the younger of the female cousins, also falls for Mr Crawford, and she is thrown into a state of jealousy when he indicates a preference for her older sister. At the same time, Edmund, who is planning a career in the Church, finds himself falling in love with Mary Crawford. Fanny, on the sidelines, observes them all closely and becomes convinced that Sir Thomas would disapprove strongly of the amateur theatricals and the various intrigues which they have inspired.
On Sir Thomas's return, Fanny's judgement is proved correct, and the play-acting is brought to an abrupt close. Maria hopes that Mr Crawford will make a formal declaration of love for her, but when he does not she marries Mr Rushworth. Meanwhile Fanny, who over time has fallen in love with her cousin Edmund, suffers great emotional pain watching him pursue his courtship of Mary Crawford. She is, therefore, quite unprepared when Henry Crawford embarks on a plan to seduce her. Initially, this is an amusing game for Henry, but he quickly finds himself genuinely in love and he approaches Fanny with an offer of marriage. Fanny refuses, deeply angering Sir Thomas Bertram, who prizes the social status and financial security that such a marriage would bring her.
During her time at Mansfield Park, Fanny has little contact with her original family, although she stays in touch with her much-loved brother William who has gone to sea as a midshipman. Henry Crawford uses his influence to obtain a promotion for William, but even this does not alter her decision to reject his marriage proposal. To show Fanny what poverty is like, Sir Thomas Bertram arranges for her to spend time with her family in Portsmouth. There she finds poverty, disorder, chaotic siblings, a drunken father and a harassed mother. She soon realises that her true home is Mansfield Park. Fanny is visited in Portsmouth by Henry Crawford, who is still determined to win her heart, and after various acts of consideration and courtesy her feelings towards him soften a little.
Meanwhile her Mansfield Park relatives are overtaken by a series of disasters and scandals. First, her cousin Tom becomes seriously ill. Then, Maria and Henry Crawford, having resumed their flirtation in London, run away together. Julia elopes with Mr Yates, the idle dilettante who had been involved with the Mansfield Park theatricals.
Once again, Fanny's judgement and good sense are revealed as her relatives come to see the truth about Henry Crawford. Edmund rejects Mary Crawford as a potential wife, having been shocked and disillusioned by her permissive attitude to her brother's behaviour. Maria is divorced by her husband and banished to live with her unpleasant Aunt Norris. Finally Edmund realises that it is Fanny who can best offer him the love and companionship he has been seeking. Fanny and Edmund marry and move into the parsonage attached to the Mansfield Park estate.