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Willoughby picks up Marianne – literally. He carries her home after she sprains her ankle in the rain.
Everyone falls for Willoughby in the Dashwood house. He becomes an instant favorite, and starts hanging out with them all the time.
Willoughby encourages Marianne to make fun of Colonel Brandon.
On the day of the failed excursion, Willoughby takes Marianne for a private drive in his carriage. He takes her to his aunt's house, Allenham, and shows her around.
Willoughby fervently declares his love for Barton Cottage (and, by extension, its inhabitants…).
The next day, Willoughby skips out on his dinner date with the Dashwoods – Elinor and Mrs. Dashwood come home earlier that day to find him with an upset Marianne. He claims that his aunt has sent him back to London, and he must obey her.
Willoughby disappears to London, and we don't hear from him for a long while.
In London, Willoughby ignores Marianne's letters. Finally, he stops by to call on them at Mrs. Jennings' house, but misses them and just leaves his card.
Willoughby, accompanied by a new girl, catches sight of Elinor and Marianne at a party. He talks to them distantly, then Marianne confronts him wildly. He's distressed, but makes no explanation.
Willoughby sends Marianne a brutally cold letter implying that she got carried away in her feelings.
We find out that Willoughby is engaged to the other woman, a certain very wealthy Miss Grey.
We learn Willoughby's real story from Colonel Brandon – the reason Willoughby was sent away from Allenham was because his aunt found out about his exploits with Eliza. He went to London to seek his fortune through a good marriage.
Willoughby and Miss Grey are married.
Willoughby encounters Sir John in town, and finds out that Marianne is deathly ill. He rushes madly to Cleveland to see her. Instead, he finds Elinor and spills his guts to her.
Willoughby confesses his true love for Marianne, and explains his desperate situation. It turns out that Miss Grey (now Mrs. Willoughby) made him write the mean letter to Marianne. He realizes that his love for her is impossible, but claims that it will never die.
Leaving, Willoughby begs Elinor to tell Marianne the truth.
After Marianne's marriage, Willoughby still admires her from a distance, though he settles into his own marriage after his period of discontent. Marianne continues to be his ideal woman.