Gary Grobowski and Brooke Meyers meet at Wrigley Field during a Chicago Cubs game and begin dating, eventually buying a condominium together. Gary works as a tour guide in a family business with his brothers, Lupus and Dennis. Brooke manages an art gallery owned by eccentric artist Marilyn Dean.

Their relationship comes to a head after the latest in an escalating series of arguments. Brooke, feeling unappreciated, criticizes Gary's perceived immaturity and unwillingness to work on improving their relationship. Gary is frustrated by Brooke's perceived controlling, perfectionistic attitude, and expresses his desire to have a little more independence, particularly when arriving home from work, wanting to unwind.

Brooke becomes irate when Gary fails to offer to help her clean up after a big dinner party at their home. Still frustrated from their earlier, unresolved argument, she breaks up with him (despite still being in love with him). Brooke seeks relationship advice from her friend Addie, while Gary goes to tell his side of things to friend Johnny Ostrofski.

Since neither is willing to move out of their condo, they compromise by living as roommates; but each begins acting out to provoke the other in increasingly elaborate ways. Gary buys a pool table, litters the condo with food and trash, and even has a strip poker party with Lupus and a few women. Meanwhile, Brooke has Gary kicked off their "couples-only" bowling team and starts dating other men in an attempt to make Gary jealous.

When their friend and realtor Mark Riggleman sells the condo, Gary and Brooke are given two weeks' notice to move out. Brooke invites Gary to an Old 97's concert, hoping that he will figure out that the gesture is meant to be her last-ditch attempt to salvage their relationship. Gary agrees to meet her there, but misses the hidden agenda, and misses the concert—unwittingly breaking Brooke's heart. When Gary goes out for a drink with Johnny, his friend points out that Gary has always had his guard up, has been guilty of a lot of selfishness, and never gave Brooke a chance, emotional intimacy-wise.

Afterwards, Brooke quits her job in order to spend time traveling Europe. When she brings a customer from the art gallery home one evening, Brooke finds the condo cleaned and Gary preparing a fancy dinner to win her back. He lays his heart on the line and promises to appreciate her more. Brooke becomes devastated and states that she just cannot give anymore, and, therefore, does not feel the same way. Gary seems to understand and kisses her before leaving. It is later revealed that Brooke's "date" (who initially asked her out, but she politely rejected) was actually a client interested in a piece of artwork she kept at the condo.

Both eventually move out of the condo. Gary begins taking a more active role in his tour guide business, while Brooke travels the world, eventually returning to Chicago. Sometime later, they meet again by chance on the street as Gary is bringing home groceries and Brooke is on her way to a meeting. After some awkward but friendly catching up, they part ways but each glances back over their shoulder and they share a smile.