You know how Shakespeare sometimes rewrote the same plot, once as tragedy, once as comedy? (The most obvious example that springs to mind is Othello/Much Ado About Nothing – one ends in death & despair, the other in multiple marriages. Another one is Romeo and Juliet vs. A Midsummer Night’s Dream.) Well, it turns out Austen did that too: Mansfield Park is a shadow plot of Pride and Prejudice.
Think about it: a young lady meets a gentleman against whom she becomes prejudiced because of the things he says and does; she is convinced he has a bad character. But he falls in love with her, and proposes. She tells him that no way, no-how, will she have anything to do with him. She then goes off on a journey; while abroad, she meets the man again, and now sees him in a very different light. Finally the journey is abruptly terminated by a family emergency: a female relative of the lady has gone off with an undesirable man, which throws her entire family into a tremendous uproar. The lady returns home, and the romantic entanglements eventually are straightened out.
Two stories, one underlying plot – once light, once dark. In the case of Elizabeth and Darcy, her prejudice turns out to have been false; she sees him truly the way he is when they meet in Derbyshire and when he prevents the ruin of her seduced sister; they get together in the end, and happily-ever-after ensues. For Fanny and Henry Crawford, it’s the opposite. Fanny’s prejudices against Henry are justified; she almost lets herself think she was wrong when she meets him again in Portsmouth, but the climax proves she was right all along. The “undesirable man” who seduces her cousin is Henry himself, and even though his crime ensures Fanny of her happily-ever-after, his, his sister’s, and Fanny’s cousin’s love lives are blighted forever.
It’s as if Austen sat down after writing P&P and thought to herself: “What if Elizabeth had been right about Darcy? What if he really was as disagreeable and bad-tempered as she thinks him at first? What if she had let herself be persuaded by how different he appeared in Pemberley to drop her prejudice and accept his proposal, and then found she was deceived?” and then she wrote out the scenario of how it could have played out.
Fortunately, the unhappy ending is only for those who deserve unhappiness – Fanny gets her man, even though it takes him a while to figure it out. But the shadow plot is there.