It is in Chapter 42 that Magwitch narrates to Pip and to Herbert the series of events that led to his deportation to Australia. He tells of how he came to fall in with Compeyson, who was in all outward aspects a gentleman. He knew how gentleman acted and could act as one of them. However, this exterior hid his true evil nature and the way he made money buy swindling and blackmailing. Provis becomes his sidekick in this business, but it is clear that at every stage Compeyson is the one in charge. Note what Magwitch says to Herbert and Pip about the relationship between them:
"...that man got me into such nets as made me his black slave. I was always in debt to him, always under his thumb, always a working, always a getting into danger. He was younger than me, but he'd got craft, and he'd got learning, and he overmatched me five hundred times told and no mercy."
Thus it is that the true mastermind behind the crimes of Compeyson and Provis was definitely Compeyson. At every stage of his narrative, Magwitch reveals the cold-hearted cunning that allowed him to be exploited and used by Compeyson, which of course foreshadows the way in which Compeyson betrays him when they are brought to justice.