It's the day of Don Vito Corelone's daughter's wedding. And like any good mob boss, he's spending his time receiving guests who are requesting favors from him.
Outside, the wedding is in full swing, and we meet a bunch of different characters, including the Don's sons, Michael and Sonny.
Don helps his lounge-singer godson, Johnny Fontane, intimidate a movie producer into giving him a part. How, you ask? Oh, no big deal. Don just has the severed head of the producer's prized racing horse placed in his bed.
That seems to do the trick.
A gangster named Sollozzo tries to rope Don and his political connections into a dope-dealing scheme with the rival Tattaglia family. But the Don refuses. (Like ya do.) And since we're in a mob movie, this leads to a near-fatal assassination attempt on Don Corleone and the murder of his trusted hit man, Luca Brasi. So Sonny becomes the acting Don, while Michael defends the Godfather from a second assassination attempt.
It's hard out there for a mob boss, eh?
Next up, Michael murders Sollozzo and a corrupt cop, Captain McCluskey, at an Italian restaurant. Mmmm...spaghetti and blood balls.
Since he just, you know, murdered two guys in cold blood, Michael flees to Sicily, where he marries a young Sicilian woman named Apollonia. Back in New York, all five of the major mafia families (Corleones included) go to war. But tragedy strikes on both continents: a car bomb intended for Michael ends up killing Apollonia, and Sonny is assassinated at a toll-booth (partly thanks to the conniving, wife-beating Carlo, his sister's husband).
So far so bad.
The Don is forced to sign a truce with the Tattaglias and agree to their drug scheme, while Michael is able to return to America. Don Corleone realizes that another Don, Barzini, was the real mastermind behind the assassination attempts against him.
Nothing like a little Don vs. Don scheming.
With the Don getting too old to handle family business, he slips into an adviser role. Michael becomes the new acting-Don, marries Kay Adams (his old girlfriend from the beginning of the movie), and begins the hard work of reassembling the Corleone family's shattered reputation. He puts pressure on the family's casino associate, Moe Greene, who risks making a deal with Barzini, and he tries to identify the traitor who's attempting to help Barzini.
Wanna guess who it is? Turns out to be Tessio, one of their main partners.
Speaking of ouch...at his sister's child's baptism, Michael becomes the baby's literal godfather. And in one of the most famous cinematic scenes ever, we flash between the baptism (aww!) and a massacre that Michael ordered (...not aww).
Michael's hit-men kill the rival Dons, including Tattaglia and Barzini, Tessio, and Moe Greene. Michael exposes Carlo as a traitor and has him killed, too.
In the end, Michael denies killing Carlo to a distraught Connie and swears the same to Kay. Kay watches as Michael's capos (under-bosses) assemble in his office, paying their respects to the new Godfather.
The door to his office closes, and the movie ends.
Make way for sequels.