The Two Timelines - First the easier Don Vito Corleone timeline:
So Vito becomes brave and stands up to the comedy mafia guy in the white suit. His friends give him their share of cash on his promise that he will get a better deal. Vito follows the old guy through the streets during the fiesta to his apartment. He breaks in to a top floor window, hides a gun in a towel, and takes him out.
Vito, Clemenza and Sal Tessio get into the Olive oil business and do very well for themselves. Vito has a tough reputation now, and people start to ask him for favours.
While on a trip to Sicily, Vito visits the ranch of the man who killed his mother and father. The old man asks who his father was - Corleone tells him and before the old man can respond to the shock he cuts him open with a knife.
--Michael Corelone timeline:
Michael needs to get to Frank Pentangeli before he can testify, but he is kept under FBI protection at an army barracks. On one of the final days of court, Michael brings Frank's brother with him into the courtroom. Frank is shaken by this - and tells the senators that it was all a lie, and he was coerced by the FBI into making a false statement. The court is adjourned and Tom demands an apology for his client.
In the hotel room after the hearing, Kay tries to leave Michael again - who is repentent - and then incites his anger by telling him that she had their baby aborted. Michael flies into a fit of rage, and tells her that he is keeping the kids.
A little while later, old Mrs Corelone dies and the family gather to pay their respects. Connie begs Michael to forgive Fredo, which he does. Tom visits Pentangeli at the barracks, and they talk about what will happen to Frank's family if he were to die and if they would be cared for - soon enough he is found dead in the bath having cut his wrists.
So now over to Hyman Roth. He arrives at an airport (back in the US?), finally allowed to enter a country after Israel kicked him out, to a barrage of press - when one of Michael's hitmen, disguised as a journalist, shoots and kills him - but is shot in the back as he makes an escape.
Fredo tries to teach Michael Jnr how to fish - but the boy is summoned to see his father. Reluctantly, Fredo gets into a boat with the hitman Al Neri and out in the lake, watched by Michael back in the house, Fredo is murdered.
It cuts back again to Vito's birthday later in life, when Michael has decided to join the Marines. Just for an opportunity to show Sonny one last time it seems as it has no real purpose.
The movie ends with Michael, alone, thinking about what his life has become.