The Man and Woman introduce Tom Ambrose, a high-powered advertising executive who is having an affair with his creative director, Jessica. Having forgotten that today is his fifteenth wedding anniversary, Tom hurries home to surprise his wife, Lucy, and discovers a lovebite on her neck. Trapped, Lucy confesses that she has taken a lover. A stunned Tom walks out into the night and, after speaking with a taxi driver, vows to confront this interloper.
As Tom spies on Lucy, the very handsome, long-haired Sebastian glides in on his bicycle and gives Lucy a passionate kiss. Tom follows Sebastian to a seedy downtown bar, where Sebastian posts a "Roommate Wanted" notice. Tom cannot resist. He approaches Sebastian and introduces himself as Milo, a man in need of a room. Sebastian, who is a destitute artist, is suspicious, but when Tom (as Milo) offers to pay the entire rent, Sebastian cannot refuse. Sebastian leads Tom to his sparsely-furnished, bohemian loft, where Tom interrogates Sebastian on every aspect of his life. Sebastian assures his new roommate that he is a contented bohemian, but Tom suspects otherwise.
A couple of days later, Sebastian has a rendezvous with Lucy, who is having doubts about their relationship, but Sebastian reassures her. Back in the loft, Tom meets their free-spirited neighbour, Edith, as Sebastian enters with the news that Lucy has lent him her husband's Porsche. Tom goes to strike Sebastian, but he puts his back out in the process. Despite Tom's objections, Sebastian gives him a massage, and the two men, for the first time, have an adult conversation about their lives. Meanwhile at the Ambrose home, Lucy unleashes her anger at her husband.
Several days later, as Tom and Sebastian work out, they begin to realise that they have much in common. When Sebastian goes to spend the night with Lucy, a despondent Tom seeks out a priest. The next morning, Tom tells Sebastian that he plans on moving out. But Sebastian wishes Tom would stay, since he feels that the two men have begun to bond. Tom agrees to remain, when the doorbell rings. It is Lucy, whom Sebastian has invited over for brunch. As Tom frantically tries to escape, everyone ponders their predicament.
It is thirty-eight seconds later. As Lucy surveys Sebastian's loft, Sebastian calls for his missing roommate, Milo. Tom, wearing a gorilla mask, enters. Lucy is delighted by this strange, masked man, while Sebastian is nothing but annoyed, and after repeated entreaties to "take off that mask," Sebastian decides that he and Lucy should leave. As they exit, Edith enters, and Tom asks her where he might be able to find a new lover for Sebastian. Edith tells him of this trendy new restaurant, and we are introduced to its Maitre d'.
Tom and Sebastian arrive; then Cindy, a model, enters. Tom has set up Cindy and Sebastian on a blind date. Although Sebastian protests, he soon relents, and Tom leaves as Sebastian and Cindy get to know each other. Feeling an intense attraction, Cindy and Sebastian kiss, but Sebastian realises that he only truly cares for Lucy, and he informs Cindy that he's not interested in her. Afterwards, Sebastian calls Lucy to tell her that he loves her, and Lucy ponders her own feelings.
That night, a defeated Tom has a few drinks in a country-western bar, and as he wallows, he plays a song on the jukebox. In desperation, Tom has called Jessica, and she arrives and tells him the truth — his marriage is over, and he should move on. Tom agrees. Tom returns to the loft to pack his bags, but as Sebastian begs him to stay, Sebastian reveals that he met Lucy when he was applying for a graphic artist's job. Tom wonders why a self-avowed bohemian like Sebastian would ever apply for a corporate job, and Sebastian reveals that things have not been going that well for him. Tom gets an idea, and he insists that there's only one way that a woman like Lucy would ever marry a man like Sebastian, and that way is for Sebastian to get a job. Sebastian protests, but Tom insists on transforming Sebastian into a graphic artist. The two men hole up in the loft in order to create a professional portfolio for Sebastian.
After a few days of non-stop work, an exhausted Tom reminisces — in a flashback — about when he first fell in love with Lucy. After they finally finish the portfolio, Tom sends Sebastian off on a make-over, and Sebastian returns sporting short hair and a power suit. An excited Sebastian goes off on a job interview, and he returns with the news that the agency was so impressed with his portfolio, they have offered him a position as creative director.
As the two men celebrate, Lucy rings the doorbell. As Tom runs off, Sebastian lets her in, but Lucy seems less than thrilled with Sebastian 's transformation. Tom overhears Lucy 's lack of enthusiasm, and, realising what he's done, he leaves. Tom returns home and tries to convince Lucy that he's a changed man, but she is not ready to take him back, so Tom returns to the one place where he always felt at home – his office. Soon thereafter, Tom is informed that he is to interview a hot new creative director. As Sebastian enters, Tom ducks behind a storyboard. Moments later, Lucy enters, looking for her husband. Though he tries to escape, Tom is soon revealed, and Sebastian and Lucy both realise the true identity of Milo.
Tom expects Lucy to be furious, but instead she is moved by the extraordinary lengths that Tom went to in order to win her back, and they agree to give their marriage one more try. But Sebastian is not happy with this arrangement. Though he and Lucy agree that their affair is over, he misses Milo, the best friend he ever had. Tom, too, misses Sebastian, and they ask if they can continue their friendship. As Lucy agrees, reluctantly, this unlikely trio contemplates what their lives now hold.