The Great Gatsby is a hard look at the decadance of the roaring 20's told by the friend of a poor boy who spent his whole life trying to be rich enough and good enough for the rich girl he fell in love with in his youth.
Jay Gatsby starts as a young war hero who falls in love with socialite Daisy. When he is back off at war, Daisy gets tired of waiting for him and gets involved in the "old money" party scene. There she meets and and eventually marries Tom Buchanan ... mostly because he has money and she can't resist the lifestyle he can provide. However, we see through the cold relationship she has with her daughter, and the shallow activities she spends her time on that her life is not at all fulfilling.
Gatsby spends years making money (through shady and most likely illegal activities) so he can impress and win back his love. He befriends his neighbor Nick (through whose eyes the story is told) and throws lavish parties near where Daisy lives for the sole purpose of luring her back into his life.
She eventually wanders into one of his parties and they re-connect and become lovers. Her husband Tom has also been having an affair with his mechanic's wife Myrtle.
All goes well for awhile, but Gatsby's "new money" (money made by himself and not inherited by his ancestors) doesn't get him accepted into high society. In fact, he is rejected by the old money people.
The big show down comes when Tom confronts Gatsby about the affair. Gatsby thinks Daisy will leave Tom and marry him, but she can't. She is too entrenched in the old money lifestyle. Daisy dashes off in a car upset by the choice she cannot make, and Gatsby follows her. Daisy ends up hitting and killing Tom's lover Myrtle...but Tom blames it on Gatsby.
In the end, Daisy reconnects with Daisy and leaves Gatsby behind, and Myrtle's husband kills Gatsby and commits suicide.
Nick goes back to the midwest, finding the east coast high society disgusting...and the sad tale ends.