The film is presented in a nonlinear narrative, jumping between various days within the 500 days of Tom and Summer's relationship. This is a linear summary of the plot.
On January 8, Tom Hansen meets Summer Finn, his boss's new assistant. Tom is 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 likes Summer. During the next few months Summer and Tom grow closer, despite Summer's telling him 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 Tom and Summer have their first argument. On day 290, Summer and Tom end their relationship after they see The Graduate, a film Summer is visibly moved by, but Tom is unaffected by. Tom does not take the breakup well, and his 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, Summer and Tom attend their co-worker Millie's wedding, where they dance together 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 to rekindle their relationship; instead, he barely gets to talk to Summer and spends most of the night alone drinking, until he spots her wearing an engagement ring. Tom leaves, close to tears. He 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 rededicates himself to architecture, making a list of firms, assembling a portfolio, and attending interviews.
On day 488, Summer sees Tom at his favorite spot in the city, and they talk. Tom expresses his confusion about her actions. Summer explains that Tom was right that true love exists, and that she discovered someone else she had all the feelings for, which she had never been sure about with Tom. Summer holds Tom's hand. She says she is glad to see he is doing well. As she leaves, Tom tells her he really hopes she is happy.
Twelve days later, on Wednesday, May 23, Tom attends a job interview and meets a beautiful girl who is also applying for the same job. He finds that she shares his favorite spot and dislike for the parking lots. Just before the interview, he makes a date to have coffee with her afterwards. She tells him her name is Autumn.

Question: What makes Tom become upset when he attends the party at Summer's apartment?
Answer: He sees that she is wearing an engagement ring.
Alternative answer: Seeing Summer's engagement ring.