A timid girl named Fanny Price comes to live with her wealthy relatives, the Bertrams, at Mansfield Park when she's just ten years old. Her own family is too big and too poor to raise her well, so her mother decides to send Fanny to live with her wealthy relatives instead. Fanny grows up very shy and is often ignored and treated unkindly by her cousins, her two aunts, and her uncle (who especially scares her).
Jumping forward many years, Fanny and her cousins are now young adults and are out on the marriage market. Financial problems force Fanny's controlling uncle, Sir Thomas Bertram, to go check on his plantation in the Caribbean. Meanwhile, a wealthy brother and sister, Henry and Mary Crawford, move to a nearby house and shake things up at Mansfield Park. Love triangles and quadrangles start springing up like crazy. Two of Fanny's cousins, Maria and Julia Bertram, are both in love with Henry; Mary Crawford tries to fall for Tom Bertram but ends up interested in his younger brother Edmund instead; and Fanny herself is in love with her cousin Edmund. A Mr. Rushworth is also on the scene as Maria Bertram's fiancé.
The young people all get into lots of trouble while Sir Thomas is out of town. After a trip to Mr. Rushworth's house, where Maria gets a little too cozy with Henry, the group starts looking for fun things to do at Mansfield Park instead. They all decide to put on Elizabeth Inchbald's play Lovers' Vows (which contains some controversial subject matter), despite the objections of Edmund and Fanny.
Sir Thomas returns home unexpectedly and makes them stop working on the play. Henry quickly departs, which breaks the hearts of Maria and Julia. Maria decides to marry Mr. Rushworth after all and leaves for her honeymoon with Julia in tow. Meanwhile, Mary Crawford starts up a friendship with Fanny, but Fanny is mainly thrilled when her brother William Price pays a visit.
Mary and Edmund fall further in love, though as a second son who will not inherit his father's estate, Edmund isn't entirely appealing to Mary. Henry decides to make Fanny his next conquest, but ends up genuinely falling for her. But when Henry proposes, Fanny refuses him, shocking all of her friends and neighbors. Fanny won't tell anyone the real reason for her refusal (she loves Edmund and thinks Henry isn't morally upright) and Sir Thomas decides to send Fanny to visit her family in Portsmouth. While there, Fanny discovers that Mansfield Park has become her real home.
Shortly after, a number of tragedies and scandals strike the family: Tom Bertram falls ill, Julia elopes, Maria has a disastrous affair with Henry and leaves her husband, and Edmund and Mary end their relationship. Eventually, Edmund falls in love with Fanny and the two marry.