Myrtle Wilson's sister Catherine and Mr. and Mrs. McKee have been invited to a little get-together thrown by Tom Buchanan at the apartment he uses for secret trysts with his lover. Once the drink starts flowing at the impromptu party, the guests become increasingly loud, brash, and vulgar. They are an obnoxious bunch who clearly have no idea of how to behave in polite society. The huge gulf in class between the denizens of the Valley of Ashes and blue-bloods like Tom is plain for all to see.
Myrtle herself starts putting on airs and graces, acting as if she's better than she really is. She thinks she's got it made now that she's gotten her hooks into a rich lover. Yet she's soon quickly and brutally put back in her box after Tom breaks her nose when she won't stop talking about Daisy. This is a sudden reminder that Myrtle's fantasies of being the next Mrs. Buchanan are precisely that. Tom sees her as a sex object and nothing more.
With the vanity of Myrtle Wilson and her entourage, Fitzgerald is showing us how the headlong pursuit of the American Dream, the desire to have money and status, to be someone in life, can corrupt ordinary people, making them act in ways they really shouldn't.