The film is presented in a nonlinear narrative; 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, living in Los Angeles. Following a karaoke night, Tom's co-worker, McKenzie, lets slip that Tom is attracted to Summer. Over the next few months Summer and Tom grow closer, despite Summer making it clear to 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 looks out over a number of buildings he likes, although the view is spoiled by parking lots. After a few weeks dating, Tom gets into a fight with a guy chatting up Summer, and they have their first argument. On day 290, Summer and Tom split up after they see The Graduate, a film which Tom thinks shows true love. Tom does not take the break up well and Tom's friends call his younger sister, Rachel, to calm him down.
Summer quits the greeting card company and Tom's boss moves him to consolations, as his depression is not suitable for happier events. Months later Summer and Tom attend the wedding of a co-worker, they dance at the wedding and Summer catches the bouquet. They sit next to each other on the journey home and Summer invites Tom to a party at her apartment for the Friday. Tom attends the party and sees that Summer is wearing an engagement ring. Realizing what is being celebrated, he leaves the party. Tom enters a deep depression, only leaving the house for alcohol and junk food. After a few days, he returns to work drunk and quits his job. He sets about re-applying himself to architecture, makes a list of firms, and begins to attend interviews.
On day 488, he meets Summer at his favorite spot and they talk; he wishes her well. Twelve days later, on Wednesday, May 23, he attends a job interview and meets a girl, who is also applying for the same job. Before entering the interview, he makes a date to have coffee with her afterward. He asks her for her name, and she replies, "Autumn."