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Elinor is justifiably horrified. What is he doing here? He begs her to stay and talk to him, saying that nothing can make him leave – he has to talk to Elinor.
Elinor agrees to speak with him, worrying that Colonel Brandon will show up while Willoughby's there…which would be supremely awkward and possibly dangerous.
First off, Willoughby wants to make sure that Marianne's out of danger – he's been worried about her. He's thrilled to hear that she's OK, and Elinor realizes that he's drunk.
Apparently, Willoughby left London that morning, and made a mad dash all the way to Cleveland, after finding out that Marianne was deathly ill.
He then goes on to explain his past actions to Elinor – he wants to tell her everything about his relationship with Marianne, and what resulted from it. Now we get their relationship from Willoughby's perspective:
When Willoughby first came to Devonshire, he viewed Marianne as a kind of plaything – like the other women he'd loved and left (such as Eliza, Colonel Brandon's ward). Because of his debts and financial dependence upon his elderly aunt, Mrs. Smith, it would have been impossible for him to marry Marianne. However, he realized that he actually did love Marianne, unlike those other girls.
Unfortunately, in pursuit of wealth, he abandoned Marianne – the only thing that could have made him happy.
Elinor softens a bit, realizing that he means what he says. Willoughby insists that the time he spent with Marianne was the happiest of his life, and he was sure that his intentions were honorable – he really meant to marry her.
At that time, though, Mrs. Smith found out about the Eliza situation, and Willoughby got in trouble. While he claims that Eliza herself was also partly to blame, he can't justify his actions. What he feels worst about is the fact that his actions with regard to Eliza also inadvertently hurt Marianne.
Mrs. Smith, who's a good, morally upright woman, told Willoughby that she would forgive him for everything if he would marry Eliza. He refused, and he was booted out of her house and her good favor.
Willoughby spent the night deliberating on his situation – in the end, his fear of poverty outweighed his love for Marianne. He left Devonshire, hoping that he would never see Marianne again, and thus never doubt his actions.
Elinor asks why he didn't call or write when they were in London; he reveals that right before he left, he did say goodbye to Marianne, and it was almost too painful for him to bear. He was totally miserable, and the thought of seeing them in London was too much.
Elinor feels bad for the poor guy, but is worried that Colonel Brandon is on his way. She pushes him to wrap up the story.
Willoughby then relates the story of the letters (that we've already read) that passed between him and Marianne in London. He was also tormented by the fact that she was there, so close, yet so far. He claims that her opinions and feelings are even more important to him than his own. It's clear that he's still madly in love with Marianne, even though he's married to someone else.
Marianne's notes reminded him of how much he loved her, and he even wavered for a moment in his resolve to leave her behind – however, his engagement to Miss Grey was settled already, and nothing could be done. He wanted to simply avoid the Dashwoods. He admits creepily that he even kind of stalked them, just to see how his beloved Marianne was doing – he was clearly desperate and miserable, and engaged to a woman he simply didn't love.
The evening of the infamous ball was the worst – Marianne on one side, and his current wife, Sophia, on the other. Then, when he found out that Marianne was dying, he was filled with desperation once more.
Willoughby realizes that he should go, and checks one more time to make sure Marianne's OK. Before he leaves, Elinor asks him curiously about the letter that he wrote in response. Apparently, he wrote it under the stern eye of his fiancée (now wife), who found out about Marianne, and forced him to copy a letter of her construction and send it to Marianne. It was Miss Grey (now his wife) who forced him to send back all of his mementos of Marianne.
Elinor makes it clear to Willoughby that it's no use trying to escape it, or trying to take it out on his wife – everything that has come to pass is his fault.
He tries to justify his complaints, saying that his wife is a terrible woman, who knew all along that he didn't love her.
Anyway, Willoughby makes Elinor promise to tell Marianne all of this once she's better. Finally, he explains how he found out about all of this – he ran into Sir John Middleton at the theatre last night, who told him that Marianne was dying of a fever at Cleveland. Sir John, bless his heart, was trying to make Willoughby feel bad in punishment for all his bad deeds; clearly, it worked.
Sir John, seeing how upset Willoughby was, felt bad for him and offered him a puppy. Good old Sir John.
Elinor tries to think over all of this astonishing information, and has no answer for him. Willoughby rises to leave – he's going home to Combe Magna before returning to town. He shakes hands affectionately with Elinor, and asks hopefully if she can forgive him, at least a little. She says that she does, and encourages him to try to be as happy as possible.
Willoughby dismisses this as impossible, and says encouragingly that if he were "at liberty again" to marry (that is, if his wife died), perhaps he and Marianne might… well, never mind.
Willoughby finally leaves, saying that there's only one thing he dreads – Marianne's marriage, especially if it's to Colonel Brandon.