Elinor and Marianne are obliged to accompany Lady Middleton to a party in town, even though Marianne is far too melancholic to enjoy dancing or card games. Suddenly, Marianne catches sight of Willoughby among the crowd and rushes forth to greet him. She is astonished and deeply distressed when he avoids her eye and appears absorbed in conversation with another young lady. When she finally approaches him directly, he coldly remarks that he indeed received her letters but never found her at home when he attempted to visit her in reply. Marianne must leave the party immediately with her sisters, for she is too overcome by grief to do anything but climb into bed.
The next day, after breakfast, Marianne shares with Elinor a letter she has just received from Willoughby. In his letter, Willoughby apologizes for anything in his conduct at the party that might have offended her. He expresses his esteem for the entire Dashwood family and regrets if he ever gave Marianne any reason to believe that he felt differently for her. Finally, he informs her of his upcoming engagement to another woman and encloses in his letter the three notes that she sent him in London.
To Elinor's dismay, all of Marianne's notes were urgent pleas for Willoughby to come visit her at Mrs. Jennings's home, even though, as Marianne confesses, they were never formally engaged to one another. Elinor can hardly believe that Marianne could be so forward in her affections when she and Willoughby were not even engaged, but she nevertheless tries to comfort her sister with gentle words, wine, and lavender drops. Marianne tells her sister that she wants to leave London immediately, but Elinor reminds her that it would be rude to leave Mrs. Jennings after such a short visit.
Mrs. Jennings tries to comfort Marianne but says all the wrong things. She remarks to Elinor that her sister looks "very bad" and that she should realize that Willoughby "is not the only young man in the world worth having." She also invites guests to dinner in order to amuse Marianne, but even her sweetmeats and olives cannot lift the girl's spirits. Marianne leaves the table early, but Elinor remains to hear Mrs. Jennings and her friends discuss how Willoughby squandered all his fortune and therefore abruptly proposed to Miss Sophia Grey, a wealthy heiress. Mrs. Jennings tells Elinor that now it will only be a matter of time before Marianne marries Colonel Brandon.
While the party takes after-dinner tea, Colonel Brandon arrives to speak with Elinor. He fears that the rumor he heard in town about Willoughby's engagement to Miss Grey might be true, and Elinor confirms his fears. The next day, he visits once again to share with Elinor the sad story of his own romantic history, in the interest of shedding light on Marianne's predicament: he explains that he was once deeply in love with a woman named Eliza, but she was married against his inclination to his brother so as to ensure her fortune for the family. Brandon's brother treated her very unkindly, and she deceived him; ultimately, the couple divorced, and she disappeared. Colonel Brandon, formerly her lover and then her brother-in-law, at last found her dying of consumption in a sponging house (a "bath," or spa) in London. He cared for her until her death and promised to take care of her three-year-old daughter. Willoughby placed the young girl in school, and she visited him periodically. Then, about a year earlier, she suddenly disappeared. The following October--the day of the intended picnic to Whitwell, which takes place earlier in the book--he received the news that she had been seduced and abandoned by none other than John Willoughby! He explains that this is why he had to rush off to London on the day of their planned outing.
Elinor shares Colonel Brandon's story with Marianne and Marianne mourns the loss of Willoughby's "good" character just as she mourned the loss of him to another woman. The sisters also receive a note from their mother expressing her shock and pain at the news of Willoughby's betrayal. Nonetheless, Mrs. Dashwood urges her daughters to stay in town, especially since their half-brother John Dashwood and his wife Fanny will be arriving there shortly. Meanwhile, Mrs. Jennings, Mrs. Palmer, Lady Middleton, and the Steele sisters also offer words of sympathy to the Dashwood sisters, though their concern is more for themselves than for Marianne: Lady Middleton, for example, expresses outrage at Willoughby's behavior but then arranges to leave her card with Miss Grey since she will be an elegant and wealthy woman when she marries John Willoughby. Only the sympathy of Elinor, Mrs. Dashwood, and Colonel Brandon is entirely genuine and well-intentioned.