Michael Corleone become the Don of the Corleone Crime Family upon the death of his father Vito Corleone. He never wanted to be part of his father's business but when an attempt was made on his life he realized the he had to avenge his father. At that point, he entered the crime family and there was no way out.
As head of the family, he became very cold and ruthless. His wife Kay who never wanted a part of that life, left him when his children were young. He had made her the promise that he would change and leave the crime family. She came to believe it would never happen, so she left. He always loved her and since she never betrayed him, he never had her killed. His brother Freddie was not so lucky. He had betrayed Michael to his enemies and an attempt on Michael's life and his family was made. When he discovered that Freddie was in part responsible, he had him killed. To Michael, once a traitor, always a traitor.
In part III, Michael has made an effort to change his life and leave the family business. He also feels deep regret for having had his brother killed and it will haunt him to the end. His children are now grown up and Vincent, Sonny, illegitimate son, wants to be part of the Carlone crime family. He wants to take over when Michael no longer can. The problem is that Michael wants out. Although Vincent loves his uncle and will do anything to protect him, Vincent is a hot head. He is a womanizer and has the same temper his father, Sonny had. He is not cold and calculated but is driven by his emotions. He will aggravate the relationships with the other crime families. He will also get involved with his cousin, Michael's daughter, against Michael's wishes.
Michael will have an opportunity to make peace with Kay and will find out that she has always love him and tells him that she will always love him.
The third movie completes the story of Michael's life. Even if he was cold and could be ruthless, he was always faithful to his family and protecting them was his primariry concern. He was alone, an outsider when, younger he did not want to join his father's business. At the end of his life, he finds himself alone again, because of the life of crime he could not longer escape.
This movie is not as good as part I and II, but it is still a must see movie.