The film is presented in a nonlinear narrative, as it jumps from various days within the 500-day span of Tom and Summer's relationship, indicated by an animation that includes the day's number; this summary is a linear version of the events of the film.
On January 8, Tom Hansen meets Summer Finn, the new assistant to his boss. Tom trained as an architect but works as a writer at a greeting card company in Los Angeles. Following a karaoke night, Tom's friend and co-worker McKenzie reveals that Tom is attracted to Summer. During the next few months Summer and Tom grow closer, despite Summer's telling Tom that she does not believe in true love, and does not want a boyfriend.
Tom shows Summer his favorite spot in the city, which overlooks a number of buildings he likes, although the view is spoiled by parking lots. After several months of dating, Tom gets into a fight with a man who flirts with Summer, and they have their first argument. On day 290, Summer and Tom end their relationship after they see The Graduate, a film which Tom thinks shows true love. Tom does not take the breakup well, and Tom's friends call his younger sister, Rachel, to calm him down. Summer quits her job at the greeting card company. Tom's boss moves him to the consolations department, as his depression is not suitable for happier events. Months later, as Summer and Tom attend the wedding of their co-worker Millie, they dance at the wedding and Summer catches the bouquet. They sit next to each other on the trip home, and Summer invites Tom to a party at her apartment. He attends the party expecting the two to rekindle the relationship, but instead spends most of the evening alone drinking (unlike his expectations of the evening where Summer spends all her time with him) until he spots Summer showing off an engagement ring and realizes what kind of party it is, at which point he leaves close to tears. Tom enters a deep depression, only leaving his apartment for alcohol and junk food. After a few days, he returns to work with a hangover and after an emotional outburst, quits his job. He decides to rededicate himself to architecture, makes a list of firms, and begins to attend interviews.
On day 488, Summer sees Tom at his favorite spot in the city, and they talk. Tom states his lack of understanding towards her actions. Summer explains that he was right about the existence of true love and that she discovered in someone else all the feelings she had never been sure about with Tom. Summer holds Tom's hand. She says she is glad to see Tom is doing well. As she leaves Tom tells her he really hopes she is happy.
Twelve days later, on Wednesday, May 23, he attends a job interview and meets a beautiful girl who is also applying for the same job. They talk, and Tom learns she shares his favorite spot and dislike for the parking lots. Before entering the interview, he makes a date to have coffee with her afterwards. When he asks her name, she replies "Autumn" and Tom breaks the fourth wall to look directly into the camera.