A subplot is a minor plot that runs parallel to the main plot, or sequence of events, in the book.
One of the most important subplots in the book is Magwitch’s situation. Magwitch is the convict Pip is accosted by in the beginning of the book. Throughout the book, we learn more and more about him. We learn the following.
Magwitch is Pip’s benefactor. Pip always thought it was Miss Havisham, but it turned out to be Magwitch all along. Pip is horrified when he finds this out, because he realizes he is not being elevated for Estella.
Magwitch is Estella’s father. We learn that Estella is the daughter of Molly and Magwitch. Jaggers took her from them because they were both in jail.
This I had strongly suspected from Provis's account … of his having kept himself dark; which I pieced on to the fact that he himself was not Mr. Jaggers's client until some four years later… (ch 51, p. 277)
Magwitch was partnered with Compeyson, who along with Miss Havisham’s brother Arthur managed to swindle Miss Havisham into marrying him and then left her at the altar.
“Young Havisham's name was Arthur. Compeyson is the man who professed to be Miss Havisham's lover.” (p. 237)
This is what caused all of her problems. If Miss Havisham had not been treated so badly, she might not have been so miserable or ruined Estella’s life.