Our narrator, Nick Carraway, begins the book by giving us some advice of his father's about not criticizing others. (But—but what if they're lying, possibly sociopathic murderers?) And now it's time to meet our cast of characters: Nick's second cousin once removed Daisy Buchanan; her large and aggressive husband, Tom Buchanan; and Jordan Baker. Jordan's a girl, and she quickly becomes a romantic interest for our narrator. Probably because she's the only girl around who isn't his cousin.
While the Buchanans live on the fashionable East Egg (we're talking Long Island, NY in the 1920's, by the way), Nick lives on the less-elite but not-too-shabby West Egg, which sits across the bay from its twin town. We (and Nick) are soon fascinated by a certain Mr. Jay Gatsby, a wealthy and mysterious man who owns a huge mansion next door to Nick and spends a good chunk of his evenings standing on his lawn and looking at an equally mysterious green light across the bay. Ookay.
Tom takes Nick to the city to show off his mistress, a woman named Myrtle Wilson who is, of course, married. Myrtle's husband, George, is a passive, working class man who owns an auto garage and is oblivious to his wife's extramarital activities. Nick, who has some good old-fashioned values from his childhood growing up in the "Middle West," is none too impressed by Tom.
Back on West Egg, this Gatsby fellow has been throwing absolutely killer parties, where everyone and his mother can come and get wasted and try to figure out how Gatsby got so rich. Nick meets and warily befriends the mystery man at one of his huge Saturday night affairs. He also begins spending time with Jordan, who turns out to be loveable in all her cynical practicality.
Moving along, Gatsby introduces Nick to his "business partner," Meyer Wolfsheim. Hm. This is starting to sound fishy. Next, Gatsby reveals to Nick (via Jordan, in the middle school phone-tag kind of way) that he and Daisy had a love thing before he went away to the war and she married Tom, after a serious episode of cold feet that involved whisky and a bath tub. Gatsby wants Daisy back, and he enlists Nick to help him stage an "accidental" reuniting.
Nick executes the plan; Gatsby and Daisy are reunited and start an affair. Everything continues swimmingly until Tom meets Gatsby, doesn't like him, and begins investigating his affairs. Nick, meanwhile, knows all about it: Gatsby grew up in a poor, uneducated family until he met the wealthy and elderly Dan Cody, who took him in as a companion and taught him how to act rich. But Dan isn't the one who left him the money.
The big scene goes down in the city, when Tom has it out with Gatsby over who gets to be with Daisy; in short, Gatsby is outed as a bootlegger and Daisy is unable to leave her husband. Everyone drives home, probably in a really bad mood, and Tom's mistress, Myrtle, is struck and killed by Gatsby's car (in which Gatsby and Daisy are riding). Gatsby tells Nick that Daisy was driving, but that he's going to take the blame for it. Tom, meanwhile, feeds Gatsby to the wolves—or at least the ticked-off husband—by telling Myrtle's husband George where to find him. Bang-bang, and George Wilson and Gatsby are both dead.
Daisy and Tom take off, leaving their mess behind. Nick, who by now has had just about enough of these people, ends things off with Jordan in a way that's about one step up from breaking up via text message. He arranges Gatsby's funeral, which is very sparsely attended—although Gatsby's dad does show up with some more info about his past. Standing on Gatsby's lawn and looking at the green light (which, BTW, turned out to be the light in front of Daisy's house across the bay), Nick concludes that nostalgia just ends up forcing us constantly back into the past.