Best friends Enid and Rebecca face the summer after their high school graduation. The girls are social outcasts, but Rebecca is more popular with boys than Enid. Enid's diploma is withheld on the condition that she attend a remedial art class. Even though she is a talented artist, her art teacher, Roberta (Illeana Douglas), believes that art must be socially meaningful and dismisses Enid's sketches as nothing more than "light entertainment."
The girls see a personal ad in which a lonely, middle-aged man named Seymour (Steve Buscemi) asks a woman he met recently to contact him. Enid makes a prank phone call to Seymour, pretending to be the woman and inviting him to meet her at a diner. The two girls and their friend, Josh (Brad Renfro), secretly watch Seymour at the diner and make fun of him. Enid soon begins to feel sympathy for Seymour, and a few days later the girls follow him to his apartment building. They find him selling vintage records in a garage sale. Enid buys an old blues album from him, and they become friends. She decides to try and find women for him to date.
Enid has meanwhile been attending her remedial art class, and she persuades Seymour to lend her an old poster depicting a grotesquely caricatured black man, which was once used as a promotional tool by Coon Chicken Inn, the fried chicken franchise now known as "Cook's Chicken", where Seymour works in a managerial position. Enid presents the poster in class as a social comment about racism, and Roberta is so impressed with the concept that she offers Enid a scholarship to an art college.
Seymour receives a phone call from Dana (Stacey Travis), the intended recipient of his personal ad. Enid encourages him to pursue a relationship with Dana, but she becomes unexpectedly jealous when he does so.
Enid and Rebecca's lives start to diverge. While Enid has been spending time with Seymour, Rebecca has found a job at a coffee shop and has become more interested in clothing, boys and other typically traditional teenage obsessions. Enid also finds herself a job so she can afford to rent an apartment with Rebecca, but she is fired after only one day. The girls argue, and Rebecca abandons the idea of living with Enid.
At the end of the summer, Enid and Seymour's lives fall apart. When Enid's poster is displayed in an art show, school officials find it so offensive they force Roberta to give her a failing grade and revoke the scholarship. Enid turns to Seymour for solace, resulting in a drunken one-night stand. Seymour breaks up with Dana, and is called to account at work when the Coon Chicken poster is publicized in a local newspaper. He unsuccessfully tries to contact Enid, only for Rebecca to tell him about Enid's prank phone call, describing the way the trio mocked him at the diner. Seymour is understandably upset and ends up confronting Josh. Seymour is injured while doing so and is hospitalized. Enid visits Seymour in an effort to apologize, causing him to realize that he has no chance of any kind of future with her.
Enid gives in to her childhood fantasy of running away from home and disappearing. Throughout the film, she has seen an old man named Norman (Charles C. Stevenson Jr.), who waits at an out-of-service bus stop for a bus that will never come. Enid sits next to him, and Norman boards an out-of-service bus. The next day, while Seymour discusses the summer's events with his therapist, Enid returns to the bus stop and boards the out-of-service bus when it arrives.

Question: Why does Seymour break up with Dana?
Answer: He has a one night stand with Enid.
Alternative answer: He breaks up with her after his one night stand with Enid.