Fanny receives a letter from Mary, teasing her about Henry's visit. Mary also mentions Edmund's presence in London in terms that make Fanny sure an engagement is imminent. No more letters arrive, and Fanny is nervous about Edmund. She sets aside her fears, though, and undertakes to improve Susan's education. Susan is a ready pupil, and Fanny is concerned that she will have to leave her behind when she returns to Mansfield Park.
Finally, a letter from Edmund arrives. He has returned to Mansfield, discouraged by Mary's behavior. Her London friends are clearly bad influences. Edmund tells Fanny that he is too much in love with Mary to think of marrying anyone else, and in his comments he mentions that he is certain Fanny will end up marrying Henry, which will make his situation even more difficult. He tells Fanny that he has seen Henry in London, and that Henry shows no signs of wavering in his devotion to her. His sisters Maria and Julia, on the other hand, are enjoying London and have no plans to return to Mansfield. Everyone misses Fanny desperately, he tells her, but Sir Thomas will not be free to come get her until after Easter. He also informs her that the Grants are going to Bath.
Lady Bertram is disappointed that Edmund has told Fanny of the Grants' trip; an inveterate letter writer, she wanted to convey the news herself. She soon has occasion to write Fanny, however. Tom Bertram, worn down by a bout of drinking, is desperately ill, and Edmund has gone to care for him. Tom's illness grows worse and worse. Finally, he improves a bit and is brought home to Mansfield. Unfortunately, the move makes him sicker, and it is feared that he will die. Easter comes and goes, and no plans are made for Fanny's return to Mansfield Park. She is disappointed that she will not be in the country to see the beauties of spring, and she is also sad not to be there to help the family.
Another letter arrives from Mary. She states quite directly that Tom's death would be a boon to her, since it would leave Edmund the heir. In her letter, she also mentions that Henry has been staying near Maria, although she claims that Fanny has no reason to be jealous. Fanny is sickened by her letter and distraught at Edmund's probable fate as the husband of such a woman, who "had only learnt to think nothing of consequence but money."
Now Edmund is the one in jeopardy. Clearly, his education has not prepared him for a woman like Mary. Sure that she has won Edmund's heart and only wanting to make certain that she gains it on her own terms, she does not hide her plottings from Fanny. In the family's time of crisis, though, and in her absence, the Bertrams have come to value Fanny's moderation and good sense. Her mentoring of Susan demonstrates her essential worth as a role model. Edmund has become a slave to his passions; rather than seeking a loving companion, he has been charmed by coquetry and flirtation. Mary and Fanny actually have something in common: their precarious upbringings have left them in situations where a good marriage would be quite helpful. Fanny, however, does not think enough of money to sacrifice her morals for it. Mary is quite willing to do so, even going so far as to wish for Tom Bertram's death to improve her situation with Edmund. Emotions, save those of friendship and familial love, are dangerous in Austen's world, for they lead otherwise good people astray. Only Fanny, who has managed to control her feeling for Edmund and instead operate on the basis of reason, is still in the right.
Again the reader is privy to hints of trouble: Henry has seen Maria again, and Mary is up to her usual tricks. At this stage in the novel, the main goal to be accomplished is the education of Edmund and his parents; that will also prove the vindication of Fanny and should provide both her and Edmund with a proper mate. Tom's illness is a signal to the family that their corrupt ways will only bring disaster, and that a change is necessary. Unsure of where else to turn, the family begins to look to Fanny, the only one among them who offers something different. This is the critical point of the novel: will Fanny prevail? Or will Mary win Edmund's heart and determine the family's direction? Mary's rather direct comments suggest that she is confident of a victory.
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.