When Dorothea arrives at the Lydgates' house, she sees Lydgate first.
He immediately thanks her for her letter and the enclosed check.
Lydgate goes upstairs to tell Rosamond that Dorothea is there.
Rosamond can't imagine why Dorothea has come back after the events of the previous day, but she can't exactly tell her husband that.
So she goes downstairs, and he leaves them alone.
Dorothea begins with an apology for the intrusion, and then tells Rosamond that she has good news about Mr. Lydgate: he's innocent, and there are plenty of people who still believe in him.
Rosamond is so touched by Dorothea's generosity that she bursts into tears.
Dorothea then feels encouraged enough to go on a bit about marriage in general. She hints about loving people other than the person you're married to, how it's hard to be strong, but that the alternative means "murdering" the marriage and then you have to live with that.
Rosamond and Dorothea understand each other, and suddenly hug each other.
And then Rosamond tells Dorothea the truth: that the day before, Will wasn't smooching her on the couch, he was telling her that he was in love with another woman.
And, she says, it was entirely Rosamond's fault, not Will's.
Rosamond hopes that Will won't be angry with her anymore and thanks Dorothea for coming and for being so sweet about everything.
Just then, Lydgate comes in again, and Dorothea leaves.
He and Rosamond make up, but things aren't as either of them imagined they would be.