Marianne awakes the next morning just as miserable as the day before. She and Elinor discuss the situation over and over again – she keeps going back and forth on the issue of how much blame to place on Willoughby.
One thing Marianne doesn't waver on is her desire to not see Mrs. Jennings, who she finds profoundly unhelpful.
Even Mrs. Jennings's good-natured idea that a letter from home will make Marianne feel better convinces the afflicted girl of her hostess's lack of sensitivity – after all, the only thing that could make Marianne feel better is the appearance of a contrite Willoughby.
To make matters worse, the letter from Mrs. Dashwood is full of questions and hints about Willoughby, since she doesn't know what has happened yet.
Marianne is more resolved to go home than ever, but Elinor convinces her to wait and see what their mother says about the situation.
Mrs. Jennings goes out on her social calls earlier than usual that day; she can't wait to tell people the news about Marianne and Willoughby.
While she's gone, Colonel Brandon shows up again. Marianne, not wanting to see him, leaves the room.
Elinor meets with the Colonel alone – he has something to tell them about Willoughby that might make Marianne feel somewhat better, or at least make her feel lucky that she didn't actually get stuck marrying him.
Finally, we get the story of Colonel's mysterious past, and his mysterious departure from Barton last fall. He takes us all the way back to a comment that he made to Elinor way early on in their acquaintance – about how Marianne reminds him of someone he once knew.
The lady Marianne reminds him of is his orphaned cousin, Eliza, who he'd loved ever since they were both kids. We get their whole tragic love history:
At seventeen, despite her love for Colonel Brandon, Eliza is married off to the Colonel's brother. The reason is this – Eliza has inherited a lot of money, and the Brandons are kind of in the lurch financially, so the Colonel's father strong-arms her into the marriage.
Colonel Brandon and Eliza continue to love each other, despite the fact that she is married to his undeserving brother. They plan to elope, but they are thwarted by a treacherous maid – Eliza is punished by her father-in-law and husband, while Colonel Brandon is sent away, first to a distant relative's house, then to the East Indies with the army.
Two years later, he hears that Eliza is divorced. At this point in telling his story, the Colonel has to pause – he's too distressed to continue. Elinor is concerned for her friend; he assures her that he's OK and continues.
Three years later, Colonel Brandon finally returns home to England. The first thing he does is seek out Eliza – it's a difficult search. Apparently, she'd been seduced by a series of bad men, and fallen into a disreputable life. From what he gathers from his brother, Eliza doesn't even have enough money left to keep her in good health.
Coincidence leads Colonel Brandon to a "spunging house," a kind of debtor's prison, where he finds Eliza, about to die of consumption (tuberculosis).
Colonel Brandon pays for Eliza to be moved to a better place to live, and tries to make her happy for the rest of her brief life.
The Colonel pauses here in his story to assure Elinor that he doesn't think the same thing will happen to Marianne – their resemblance will surely not lead to similar fates.
Back to his story: When Eliza dies, she leaves Colonel Brandon in charge of her only child, a little girl who was born out of wedlock after her first affair. The child is only about three years old. It turns out that this is the girl that Mrs. Jennings thought was Colonel Brandon's love child – in fact, she's his ward (he's her legal guardian).
Colonel Brandon provides the little girl (also named Eliza) with schooling and then with a caretaker in Dorset, where she lives with a bunch of other girls about her own age (she's seventeen now).
Last February, though, Eliza disappeared. The Colonel had given her permission to go to Bath with a friend and the friend's family – and from there, she ran off somewhere. Nobody could or would say where – and she was gone for eight mysterious months.
Elinor realizes that this, unfortunately, is probably where Willoughby comes in.
Colonel Brandon goes on. Last October, when he had to flee from Barton, he'd received news of Eliza – actually, a letter from the girl herself. He confirms the fact that Eliza's disappearance was Willoughby's fault – he'd seduced her, left her, promised to return, but then never did. Basically, he got her pregnant and fled the scene.
Elinor is shocked and dismayed. Colonel Brandon explains that this is why he's been so worried about Marianne's relationship with Willoughby – he didn't want to interfere, and he'd thought that maybe Marianne could reform him. However, it's clear that she couldn't.
Colonel Brandon hopes that Marianne can at least feel grateful that she's not in the same situation as poor the poor young Eliza.
Elinor asks if Colonel Brandon has seen Willoughby since this all went down. Apparently, he challenged the younger man to a duel, but both of them emerged unscathed.
Elinor then inquires about Eliza's state – and learns that the girl, who just had her baby, has been moved to the country.