(500) Days of Summer is presented in a non-chronological format, each scene being introduced by which of the 500 days it is. The plot as given here has been rearranged in chronological order. It is important to note cinematographically that the film opens with day 488, the scene where Tom and Summer are seated at a bench and the audience sees Summer's ringed finger as the two hold hands.
Tom (Joseph Gordon-Levitt) works at a greeting card company as a card writer. He has a few friends and is seemingly happy with his life. One day at a meeting, Tom's boss (Clark Gregg) introduces the board room to Summer (Zooey Deschanel), his new assistant. He is immediately taken by Summer's beauty.
Tom talks to his friends about how much he likes Summer. While at first he plays things cool, he is convinced she is "the one" after an exchange in the elevator over their mutual love of the Smiths. He spends two weeks pining over her and executing awkward attempts to initiate conversation, but can't hit a chord. An opportunity arises when Tom's best friend tells him that the entire office is going to a karaoke bar the following evening.
Tom arrives at the bar as his best friend is singing. He sees Summer and she seems happy to see him. Summer goes up and sings while Tom drinks with his friend and watches Summer. Summer then gets Tom a little drunk and has him sing karaoke. Afterwards, the three sit together and talk about relationships and love, during which Tom and Summer argue over whether or not love is real, with Summer saying it isn't while Tom says it is. The two agree to disagree.
While helping Tom's friend to a cab, Tom's friend blurts out that Tom likes Summer and the two are left on the sidewalk. Summer asks if it is true and Tom, after some coaxing from Summer, says that he likes her (but adds "as friends"). Summer calls him "very interesting" and leaves Tom standing there on the sidewalk. The next day at work, she kisses him in the copy room. Once Summer's copies are finished, she simply ignores Tom and leaves the copy room.
Summer and Tom go out and have fun in the city. As they spend more time together, they become closer. Summer shares her most intimate thoughts and stories with him, while Tom takes her to his favorite spot in the city and tells her about how he was studying to become an architect before he ran out of money and was forced to work for the Card Company to sustain himself. As the two walk around Ikea, they joke about living as a married couple in the store's demonstration rooms, but as they're about to kiss on a bed in a store, Summer makes it clear that she isn't looking for anything serious.
They return to Tom's bedroom and start to make out on his bed. He goes into the bathroom and convinces himself that it's just casual fun, reminding himself to just take it slow. As he walks out, however, he sees her lying naked on the bed. They presumably make love, although it is never shown.
He walks out the next morning, dancing and upbeat. He's in love with Summer and on top of the world. After a singing and dancing scene, Tom arrives at work and starts suggesting high quality slogans for cards, all the while thinking of Summer.
Things go well for a few weeks. However, one night in a bar, a random guy starts hitting on Summer. She makes it clear she isn't interested, but he won't believe she's with Tom. After taking some more insults, Tom stands up and punches the guy. He awkwardly smiles at Summer before the guy gets up and beats him down. She takes Tom to her apartment and yells at him since she thought he was not acting cool at all and did it for his own sense of self. Tom starts fighting back. He yells that everything they are doing isn't a friendship, and that even though she doesn't want to label things, he thinks that they're in a relationship because Summer isn't the only one who gets a say. He storms out.
The two go to bed. Tom contemplates calling her, but doesn't. She comes to his apartment and apologizes. The two have a heart to heart and share their past relationships. She tells him about her bisexual experience, her first relationship and her relationship with "The Puma". The two seem happy.
Weeks pass. Summer and Tom go out and see a film. She starts crying as she sees the ending of The Graduate, as Dustin Hoffman and Katharine Ross escape a wedding and their looks of joy and excitement slowly vanish. Summer tells Tom that she's exhausted and wants to sleep, but Tom convinces her to go get pancakes with him. Whilst waiting for their order, Summer blurts out that she thinks they should stop seeing each other. Tom is taken aback and asks why if they were so happy. Summer says that she isn't happy. As the pancakes arrive, Tom stands up and leaves. Summer tells him not to leave because he is still her best friend.
Tom's friends call his little sister, Rachel, who bikes to his apartment. Tom is upset and she makes him explain what happened. He recounts the break up. He is adamant that he doesn't want to get over her, but get her back. His depression worsens and begins to affect his work. His boss calls him in and he asks if Tom's performance is related to Summer leaving. Tom tries to play it off, but his boss tells him that everyone knows. It is made clear that Summer also quit her job at the card company.
Months pass. One of Tom's co-workers is getting married. He takes the train to go to the wedding and sees Summer on the way to his seat. He tries to hide, but she sees him and approaches. They talk and go get a coffee. The entire weekend is spent together. At the wedding, he asks her to dance and they have a very romantic night together. She invites Tom to a party on at her place. Tom is hopeful.
Tom's expectations for the night are to make his dreams a reality. However, as he arrives at the party, the differences between his dream night and that night are hauntingly apparent. He barely talks to Summer and finds comfort with the bottle. He then sees Summer showing off her ring and her fiance. Tom loses it. He leaves without a word, angry and hurt that Summer would treat him like this.
Tom spends the next 2 days in a catatonic state. He leaves the apartment once to buy orange juice, Twinkies and Jack Daniel's. He eventually returns to work, just in time for the weekly presentations. His best friend reminds him that today is the day they present their pitches for new cards. Halfway through the presentation, Tom decides that his beliefs of love, fate, and relationships were wrong. He gives the board a passionate speech about how their company is comfortable feeding people lies and that they are the reason people have such unrealistic expectations. He quits and leaves. The board is left stunned as his best friend awkwardly claps at his friend's departure.
Tom is seen struggling with his depression. He slowly begins to take steps to help himself. Rachel tells him that he should take a second look at Summer and Tom's relationship and stop ignoring the bad. He realizes that there was always something Summer was holding back and kept her from truly being "in" the relationship. He begins to take up architecture again. He trashes his apartment and begins drawing on the walls and designing. He slowly builds up a portfolio and makes a list of firms to present his work at. One by one he is rejected. Once the firms dwindle to a handful, Tom begins to lose hope.
Tom goes to the bench at his favorite part of the city. He's gazing down at some of the buildings when Summer calls out to him. She compliments his looks and tells him she knows he's angry. She also tells him that she's happy to see he's doing alright. Tom confesses that he now realizes that all his ideas about love were wrong.
Summer points out that they weren't. The girl who didn't want to be anyone's girlfriend was now someone's wife. She tells him that with her husband, she knew what she was never sure of with Tom: that she was in love with him and wanted to spend the rest of her life with him. She also tells Tom that if there's no destiny, she might have easily never randomly met him in a deli. She tells Tom he was right, but just not about her. She holds his hand and squeezes before telling him that she needs to go. Tom tells her he's happy that she's happy, but makes a point not to congratulate her on her marriage. She smiles and leaves Tom.
Tom is on his way to a job interview. As he sits there, waiting to be interviewed, a girl across the waiting room calls out to him and asks if they've met. She tells him that she's seen him sitting on a bench in Tom's favorite spot, which is coincidentally her favorite spot as well. Tom says he's never seen her before, so she replies he probably was not looking.
Tom jokes that since she's the competition he hopes she doesn't get the job. She returns the wish. The interviewer calls Tom in, but as he's walking he turns back and asks the girl to get some coffee afterwards. She tells him that she is meeting someone. However, as Tom turns around, she agrees and says they'll work it out.
Tom smiles and introduces himself. She smiles and introduces herself. Her name is Autumn.
Then, on the screen, it reads "Day 1", signifying that this is the first day of his relationship with Autumn.