It has been over 20 years since I have seen this movie. I remembered three of the famous scenes, "Don" Vito Corleone holding court before the wedding party, Sonny and the girl humping against the door, the horsehead in the bed. However, I was amazed at the sense of pageantry at the opening wedding scene. It would cost a gazillion dollars to try to create a movie with this level of detail today. I think the scene spans between 1945 - 1955. Coppola does a fantastic job of illustrating the best parts of traditional Italian life.
Yes, this is a terribly violent movie, but it could worth just watching the opening wedding scene if you have streaming video.
I am not going to summarize the plot, I think Wikipedia does a better job that IMDB in this case.