OK, let's back up a bit to see what else is happening.
A day before Bucket explained the whole deal, Mrs. Bagnet went up to Chesney Wold and found Mr. George's mom, who turns out to be... Mrs. Rouncewell!
She's totally psyched and has already forgiven George for not even writing all these years. She didn't even know if he was alive or dead!
Anyway, now Mrs. Bagnet fills her in on how she found her (through various hints that George dropped) and that Mrs. Rouncewell needs to convince him to get a lawyer.
There is a tearful reunion in the jail.
Dickens is really good at these scenes. Shmoop might have needed a handkerchief.
George agrees to a lawyer and anything else his mom wants. His only condition is that he doesn't want his brother the Ironmaster knowing about him. He's embarrassed that he hasn't amounted to anything.
Mrs. Bagnet takes Mrs. Rouncewell home to the Dedlock place in London. It's now the morning of Bucket's big reveal.
Mrs. Rouncewell takes Lady Dedlock aside and tells her: 1) her missing son was just found, 2) he's in jail for the murder of Tulkinghorn, and 3) Mrs. Rouncewell is sure that Lady Dedlock has some information that could free him because 4) a letter just arrived at Chesney Wold saying "LADY DEDLOCK MURDERESS" on it.
Lady Dedlock kind of collapses in her chair, and Mrs. Rouncewell leaves the room.
In comes Guppy. He tells Lady Dedlock: 1) he hasn't been pursuing the mystery, but 2) Smallweed and the Chadbands have, and now 3) they are talking to Sir Dedlock and trying to blackmail him.
A footman tells Lady Dedlock that, yes, people like that have been in the house, and now Sir Dedlock is alone in his study.
Lady Dedlock freaks out – super-duper total breakdown, completely loses it.
Then she writes a good-bye letter to Sir Dedlock, telling him she's guilty of everything except Tulkinghorn's murder. She leaves all her belongings behind and runs off.