The big exposition in chapter 42 is that the man who betrayed Magiwtch, Compeyson, also betrayed Miss Havisham.
A big secret is revealed in chapter 42. Magwitch has just shown up in London, and Pip knows that he is his benefactor. He says he is not going to tell his whole life story, but he does tell Pip why he wanted to create a gentleman.
“Young Havisham's name was Arthur. Compeyson is the man who professed to be Miss Havisham's lover.” (Chapter XLII, p. 237)
This sentence is not revealed until the last line of the chapter. It ties everything together, and brings Pip’s life—and the plot of the book—full circle. We learn that Compeyson betrayed Miss Havisham, and this is what turned her crazy and caused her to ruin Pip and Estella’s lives.
Magwitch's life is unknown region in the book. He was a juvenile offender who from the time he became aware of himself till now has been a life of crime. In the course of his life of crime he was introduced to Compeyson a persuading, handsome conman. Compeyson's accomplice was Arthur Havisham, the disinherited stepbrother of Miss Havisham. Arthur along with Compeyson had cheated Miss Havisham of her money and most importantly Compeyson had cheated Miss Havisham of her money by pretending to be in love with her and promising to marry her. Arthur who was staying at the top of Compeyson's house was now dying and soon Magwitch becomes Compeyson's partner in crime. Shortly Arthur dies screaming that the ghost of Miss Havisham is killing him.
This is the chapter where Provis aka Magwitch narrates to Herbert and Pip his life-story. The readers come to know that he is more or less a victim of circumstances at first and his companion Compeyson afterwards. A very important revelation comes with this narration as we come to know that it is Compeyson who had pretended to be Miss Havisham's disloyal lover. Her half-brother, Arthur was also an associate of Compeyson in this act.