The letters that Rochester receives give him details about her family and her past. Like the fact that her mother was mad, and she apparently tried to kill her husband. Antoinette is headed down the same path.
After receiving the third letter, Rochester is understandably scared at this point, and he goes to see the person who wrote the letters. His name is Daniel Cosway, and her claims to be Antoinette’s half brother.
However, when he arrives at Daniel’s residence, Daniel is immediately mad at Rochester for not replying to the letters, because that implied to Daniel that Rochester didn’t believe him. Well, what was Rochester supposed to say? Daniel told Rochester he couldn’t do anything about his sister. And, Daniel wanted 500 pounds because he says their father hadn’t given him anything. Even more freaked out, Rochester leaves.
Meanwhile, Antoinette is hurt that Rochester isn’t spending time with her anymore. So she goes to Christophine for help. Christophine thinks Antoinette should just leave him because he never loved her in the first place. But that’s the last thing Antoinette wants to do; she wants to make her marriage work. After some arguing, Christophine finally agrees to help Antoinette get Rochester back.
From this point on, everything is just one giant mess. Christophine puts some kind of homemade recreational drug into Rochester’s drink. However, the salvation of the marriage doesn’t happen because Antoinette’s madness is taking over, and Christophine has to keep her drugged so she won’t go crazy. It is at this point that Rochester starts calling Antoinette by her middle name, Bertha. I think this emphasizes the fact that he no longer recognizes the woman her married as his wife because she has gone mad.
Before Rochester leaves for England, Christophine makes a request. Why doesn’t Rochester leave Antoinette here with Christophine and give her inheritance back? This doesn’t work on Rochester because he has never trusted Christophine; he blames Christophine for Antoinette’s current state.
So, Rochester takes Antoinette back to England and hires Grace Poole to take care of her because he doesn’t want to deal with her. He’d rather forget that she ever existed. The last scene overlaps with Jane Eyre; it is the first fire from Antoinette’s point of view.