Michael and his live-in girlfriend Jenna appear to be enjoying the perfect relationship: Jenna is pregnant prior to the opening of the film and her parents are pressuring the pair to get married. Jenna claims that Michael's work pressures and her own dissertation rendered it an inopportune time for marriage. The real reason, unbeknownst to anyone, is that Michael feels trapped and scared. Although he considers Jenna an ideal companion, Michael is harboring second thoughts.
Upon a chance encounter at a wedding, Michael meets Kim, whom he confides in about his relationship. Kim guesses he is about to break up with Jenna and becomes flirtatious. While Michael becomes intrigued by Kim's youthfulness, openness, and spirit, he does not succumb to temptation. The two part ways with Kim telling Michael where she attends school and where she usually hangs out.
Michael eventually seeks her out at the Memorial Union but tells Kim he was in the area only because of a client meeting. Kim senses his interest and, while Michael drives her home, invites him to a party. Michael accepts. Back in the office, Michael constructs an excuse to be away from Jenna on the night of the party. He asks Chris, his friend and co-worker, to cover for him in case Jenna calls. Chris suspects Michael has met another woman and wishes to avoid becoming involved, as Chris himself has just left his wife; he knows first hand how painful a breakup can be. Michael denies the existence of another woman and merely says he will 'be with an old college friend.'
After the party with Kimwho Michael says makes him 'feel ten years younger'the two kiss several times, preceding Kim inviting Michael to her dorm for the night. Guilt over cheating on Jenna prevails over temptation, however, and he refuses. Unfortunately for Michael, Izzy's father (Izzy is a mutual friend to Michael and Jenna) dies that night. Several friends and acquaintances, including Jenna and Chris, go to Izzy's home with their condolences. There, Jenna realizes Michael had not been, as he claimed, with Chris. Jenna confronts Chris but Chris refuses to answer her questions, which only fuels Jenna's suspicions that Michael has indeed gone astray.
When Michael arrives home that night, Jenna becomes confrontational; Michael at first denies he was out with Kim, but eventually admits he had seen another woman. Although he points out he didn't sleep with her and that the outing meant nothing, Jenna yells at him and kicks him out of the house.
Alone, depressed, and desperate, Michael receives a call from Kim. Kim apologizes for being demanding earlier and asks him to come over for conversation only. Michael agrees. Upon arrival, the pair immediately engage in sexual intercourse. The next morning, Michael tries sneaking away without waking Kim but notices once he gets out the door that he left his keys behind. Upon his return, Kim demands to know why he hadn't said goodbye. Michael tells her simply that he did not want to wake her, as he had to be at work early. Kim takes the missing keys out of her pocket and returns them after Michael promises to call her.
At work, Michael plans on leaving early to seek out Jenna. On his way out, Kim visits his office unannounced, wanting to give him a mix CD. Michael confides he still loves Jenna who was, in fact, pregnant with his baby. He apologizes to Kim for not telling her about Jenna's pregnancy and leaves her in search of Jenna.
Michael pulls up to Jenna's parents' home, and Jenna's father gives a stern lecture about commitment and adulthood and offers advice on winning Jennas forgiveness. The father urges Michael to be completely honest and never stop trying. Equipped with his wisdom, Michael goes into Jenna's room. On the verge of reconciliation, Jenna asks if he was telling the truth about having sex with Kim. Michael says he was telling the truth (which he was, at the time), but as a result of her father's advice he confesses he went back later that night. Even though Michael claims he was only being honest, Jenna becomes inconsolable and storms out of the house and back to their apartment.
Michael follows her back and finds himself locked out. He stakes out on the front porch until Jenna agrees to talk. Both day and night, wet and dry, Michael remains at the front door with many neighbors taking notice and some even providing beverages to him. Jenna's father even proceeds to drive by in his car and notices Michael, who sees him. A proud smile develops on his face as he drives off, indicating that Michael has taken his advice seriously. Slowly but surely, she begins to relent, first tossing out a blanket during a cold evening, then dropping off a sandwich the next day. During the evening of what would have been his third night on the porch, Jenna breaks her silence and speaks to Michael through the closed door. She laments about mourning the loss of the relationship like the loss of someone's life. Later that evening, Jenna opens the door and Michael goes inside.