Chapter 39 is the chapter you are referring to, I think. In this Chapter the mystery concerning the identity of Pip's benefactor is cleared up. This chapter narrates how one stormy night the convict from the marshes (in Chapters 1-5) turns up at Pip's place and declares to Pip that he is the benefactor behind Pip's great expectations in return for the help he received from Pip but also for his own ends - to create a gentleman.
This revelation causes Pip to feel disgusted, disappointed and also incredibly guilty. Any hope of marrying Estella is now gone and he realises with remorse that he has abandoned his true friend, Joe, as a result of what a criminal has done. Pip feels forced into giving Magwitch shelter to protect him.
This is clearly the climax of the novel. It is worth noting how Magwitch can be compared to Miss Havisham, in that both have "parented" a child with mixed motives. Revenge is clearly something both hope to achieve through their "children", Pip and Estella. Also look for links comparing Chapter 1 and Magwitch's sudden appearance from behind the grave of Pip's father (very symbolic) with the weather and his appearance in this Chapter. Pip's guilty association with crime which began in Chapter 1 and has been repressed ever since is now made explicit. The rest of the book details how Pip copes with this revelation and changes as a result.