Using flashbacks, but narrating some of the events in the present tense, Nick Carraway recounts the events of his life after he has returned from the war and moved to West Egg. The events revolve around Nick's neighbor, Jay Gatsby, and Gatsby's attempts to win back the girl of his dreams: Daisy Buchanan.
The story is told from Nick's perspective so the plot has a lot to do with Nick's perceptions about Gatsby. Early in the novel, Nick establishes himself as an honest person and he is therefore a relatively objective and reliable narrator.
The plot begins with Nick moving to West Egg. He pays a visit to his cousin Daisy Buchanan and meets her husband Tom and their friend Jordan Baker. Nick then meets Gatsby and begins to learn more about him. Gatsby lives in a much larger house. Gatsby hosts parties that are well attended and Gatsby seems to be a very popular man in high society; however, he has no real close friends and no one really knows much about Gatsby or his past.
Through the course of the novel, Nick discovers that Gatsby and Daisy were in love in their youth. Gatsby did not have enough money to marry her. By the time he came back from the war and began to accumulate some wealth, Daisy had already married Tom (a marriage of love and financial convenience). Gatsby still intended to pursue Daisy in any way he can, so he moved into the house across the water from Daisy's house. James Gatz reinvented himself to become Jay Gatsby; he believed he needed to become wealthy and successful so that he would be good enough for Daisy. He hosted elaborate parties to make a name for himself in high society in order to ingratiate himself in Daisy's social circles.
Knowing Nick and Daisy were cousins, Gatsby is able to stage a reunion and a spark reignites between Daisy and Gatsby. Trouble ensues because Daisy is married. Matters are complicated because Tom is also cheating on Daisy. With all of the lying and cheating going on, the plot reaches its height when Tom, Daisy, Gatsby, Nick, and Jordan spend the day in the city. The woman Tom is having an affair with, Myrtle, is killed in a car accident (Daisy is driving Gatsby's car). Myrtle's husband comes after Gatsby, killing him and then himself.
A major theme behind the plot is the corruption of the American Dream. Gatsby came from poor beginnings. In attempts to win the heart of the rich, beautiful girl, he felt he needed to become wealthy: something he achieved through illegal means. The corruption extends to the culture of Tom and his social circles. The people who attend Gatsby's parties are superficial and Daisy's and Tom's marriage is based on lies and deception. The deaths at the end of the novel reinforce this idea that attempts to achieve the American Dream in spite of this culture of corruption and lies will end in tragedy. Another theme underlying the plot is trying to relive the past. Fitzgerald sums this up in the last lines of the novel:
Gatsby believed in the green light, the orgastic future that year by year recedes before us. It eluded us then, but that’s no matter—to-morrow we will run faster, stretch out our arms farther. . . . And one fine morning——
So we beat on, boats against the current, borne back ceaselessly into the past.