For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Hidy-ho, Neighbor
Nick meets his party-hardy next-door neighbor, the immensely wealthy Jay Gatsby, who has a suspicious past and a suspicious access to illegal alcohol. Turns out, that suspicious past also includes a flight with Nick's distant relative Daisy, now married to a belligerent and wealthy Yalie. The first act ends when Nick arranges a meeting between Jay and Daisy.
Sparks aren't the only thing that fly.
Act II
Lifestyles of the Rich and Famous
Daisy and Gatsby resume their love affair. Tom isn't too pleased, despite the fact that he has himself a little bit on the side. After digging into Jay's past, Tom reveals the shocking truth one hot night: Jay is a bootlegger. Gasp! Rich people are rich; do nasty things to each other.
Act III
Here for a Good Time, Not a Long Time
Daisy returns to Tom, obviously, because she's old money and Gatsby is new money, and new money is okay if you're attending its fabulous parties—but you aren't actually going to leave your society husband to marry it. On the way home from New York City, Gatsby's car hits and kills Tom's mistress. Tom isn't too upset, but the woman's husband is. He murders Gatsby; Tom and Daisy flee back west, and Nick stays behind to pick up the pieces. And apparently write a book.