A young girl named Fanny Price comes to live with her wealthy uncle and aunt, Sir Thomas and Lady Bertram. Fanny's family is quite poor; her mother, unlike her sister Lady Bertram, married beneath her, and Fanny's father, a sailor, is disabled and drinks heavily. Fanny is abused by her other aunt, Mrs. Norris, a busybody who runs things at Mansfield Park, the Bertrams' estate. The Bertram daughters, Maria and Julia, are shallow, rather cruel girls, intent on marrying well and being fashionable. The elder son, Tom, is a roustabout and a drunk. Fanny finds solace only in the friendship of the younger son, Edmund, who is planning to be a clergyman. Fanny grows up shy and deferential, caught as she typically is between members of the Bertram family.
Sir Thomas leaves Mansfield Park for Antigua, where he owns plantations. In his absence, two new figures arrive at Mansfield: Henry and Mary Crawford, the brother and sister of the local minister's wife. Henry and Mary are attractive and cheerful, and they soon become indispensable members of the Mansfield circle. Henry flirts extensively with Maria, who is engaged to marry the boring but wealthy Rushworth. He also flirts with Julia when it suits his purposes. At first, Mary is interested in Tom, the older son and heir, but she soon realizes that he is boring and not really interested in her. She finds herself increasingly attracted to Edmund, although the prospect of marrying a clergyman does not appeal to her, and she is often cruel to him on this account. In the meantime, Fanny has innocently fallen in love with Edmund, although she does not even admit this to herself. Yates, a visiting friend of Tom's, proposes that the group should put on a play. His idea is eagerly received by all except for Edmund and Fanny, who are horrified at the idea of acting. The play goes on anyways, however; Maria and Henry, as well as Mary and Edmund (who has been prevailed upon to take a role to avoid bringing in an outsider to play it), get to play some rather racy scenes with one another. When one of the women cannot make a rehearsal, Fanny is pressured to take a role. She is almost forced to give in when Sir Thomas makes a sudden entrance, having arrived from Antigua.
Sir Thomas is unhappy about the play and quickly puts a stop to the improprieties. Since Henry has not declared his love, Maria is married to Rushworth. She and Julia leave Mansfield Park for London. Relationships between the Crawfords and the Bertrams intensify. Edmund nearly proposes to Mary several times, but her condescension and amorality always stop him at the last minute. He confides his feelings to Fanny, who is secretly upset by them. In the meantime, on a lark, Henry has decided to woo Fanny. He is surprised to find himself sincerely in love with her. Fanny has become indispensable as a companion to her aunt and uncle, and on the occasion of her brother William's visit, they give a ball in her honor. Some time after the ball, Henry helps William get a promotion in the Navy. Using this as leverage, he proposes to Fanny, who is mortified and refuses. He continues to pursue her. Her uncle is disappointed that she has refused such a wealthy man, and, as an indirect result, she is sent to stay with her parents in their filthy house. Meanwhile, Edmund has been ordained and continues to debate over his relationship with Mary, to Fanny's dismay.
Henry comes to see Fanny at her parents' and renews his suit. He then leaves to take care of business on his estate. Fanny continues to receive letters from Mary encouraging her to take Henry's proposal. A series of events then happen in rapid succession: Tom Bertram falls dangerously ill as a result of his partying and nearly dies; Henry, who has gone not to his estate but to see friends, has run off with the married Maria; Julia, upset over her sister's rash act, elopes with Yates, Tom's friend. Fanny is recalled to Mansfield, bringing her younger sister Susan with her. Edmund has finally seen through Mary, who has admitted that she would like to see Tom die so that Edmund could be heir, and who has more or less condoned Henry and Maria's actions. He is heartbroken, but Fanny consoles him. Maria and Henry eventually split, and she goes to the Continent to live with the evil Mrs. Norris. Julia and Yates are reconciled to the family. Edmund finally comes to his senses and marries Fanny, and Susan takes her place with the Bertrams. Edmund, Fanny, and the rest of those at Mansfield live happily, while Henry, Mary, and Maria are cast out.
Regarding Maria Bertram and Mrs. Norris in the last chapter, neither leaves England. When Austen writes that "an establishment [is] being formed for them in another country," she does not mean continental Europe. Here, "country" simply means another part of the same country (most likely somewhere in the countryside). They are still in England.