Will is determined to see Dorothea one last time before he leaves Middlemarch, even though he already said his "final" farewell the last time he saw her.
But her well-meaning brother-in-law, Sir James, doesn't like that Will is lingering around Middlemarch, and has heard that he's been spending an awful lot of alone time with Rosamond Lydgate.
He wants to warn Dorothea that she really shouldn't see him again, since he's obviously got a thing for married women, and he gets Mrs. Cadwallader to pass on the gossip.
Mrs. Cadwallader, of course, is only too happy to oblige.
Dorothea doesn't want to believe it, and angrily leaves Fres***t to go to her uncle's at Tipton.
When she arrives, Will is there (he was picking up some sketches he'd accidentally left behind).
He tells her he's leaving for real this time, and that it's because of the codicil – it's an insult to his honor, and he can't stick around.
She assures him that she never thought badly of him, codicil or no codicil.
He comes pretty close to telling her he's in love with her, but only by saying that he'll have to live without hope, because he could never ever act on his feelings. That would make him look like he's only out for her money, and he's too proud and honorable for that.
They say goodbye again, and Dorothea's happy, rather than sad – happy that she was right to believe in him, and happy that he loves her.
She understands the motives that are driving him to leave, and she's sorry that she can't help him at all.
He's less happy about the whole thing, of course, because he's not sure that she loves him.