The Godfather "Don" Vito Corleone is the head of the Corleone mafia family in New York. He is at the event of his daughter's wedding. Micheal, Vito's youngest son and a decorated WW II Marine is also present at the wedding. Micheal seems to be uninterested in being a part of the family business. Vito is a powerful man, and is kind to all those who give him respect but is ruthless against those who do not. But when a powerful and treacherous rival wants to sell drugs and needs the Don's influence for the same, Vito refuses to do it. What follows is a clash between Vito's fading old values and the new ways which may cause Micheal to do the thing he was most reluctant in doing and wage a mob war against all the other mafia families which could tear the Corleone family apart.

The life of Vito Corleone is shown as he becomes from a boy born in Sicily to one of the most respected mafia dons of New York while Micheal attempts to expand his business empire into Las Vegas, Florida and pre-revolution Cuba while facing his own personal problems trying to keep his collapsing marriage and relationship with his brother intact.

It is 1979, about 22 years since the events of The Godfather II. For Michael Corleone, the move to legitimacy is complete: the New York crime business has been handed over to Joey Zasa and all elements of the Corleone business empire are legal, non-criminal enterprises. Michael, approaching 60, is now thinking about his legacy. His charity, run by his daughter Mary, has just handed over $100 million to the Catholic Church. Michael also intends buying a large stake in International Immobiliari, a Vatican-run property company. Things are peaceful and stable but then Vincent Mancini, Sonny Corleone's illegitimate son, starts a feud with Joey Zasa. This has far-reaching, deadly consequences, including for Michael's deal with the Vatican.