Matt Sullivan, played by Josh Hartnett, resides in San Francisco with his roommate Ryan. He works at a dot-com company but finds himself tormented by obsessive thoughts of his ex-girlfriend, Nicole, portrayed by Vinessa Shaw. This fixation leads to sexual dysfunction that he confides to his brother John, a Catholic priest-in-training played by Adam Trese. After a comically disastrous attempt to fake an orgasm with a date named Susie, it’s revealed that Nicole is now engaged after meeting someone in France. In a bid to regain control over his life, Matt decides to abstain from all sexual activity—including masturbation—for the 40 days of Lent.
On the very first day of this self-imposed celibacy, he clears his apartment of anything that could remind him of Nicole. While at a laundromat, he meets a stranger named Erica, played by Shannyn Sossamon. Though Matt struggles to engage in conversation, an unexpected connection begins to form between them. However, unbeknownst to Matt, his friends and coworkers start an office pool, betting on how long he can resist temptation, a competition that catches on quickly, even spreading online. Fast forward to day 11, and Matt remains steadfast in his commitment, leaving both his brother John and Ryan amazed.
As time progresses, it becomes apparent that this betting pool garners more attention, particularly from women attempting to incite Matt's sexual urges. One bold participant, Candy, even photocopies her backside to lure him. In his interactions with Erica, it is revealed that she works as a “cyber nanny,” filtering out internet pornography, and their chemistry becomes undeniable. They embark on a date on the city bus, but Matt awkwardly avoids any physical connection, leading Ryan and the others to speculate on whether Matt genuinely likes her.
Their relationship takes a hit when both discover the existence of the betting pool, leaving Erica understandably upset, believing Matt and his friends had reduced their budding romance to a mere wager. As the days add up, Matt's boss surprisingly joins him in his celibate phase. Even as Matt's feelings for Erica deepen, he struggles with the emotional toll of his vow, especially when he unexpectedly encounters his former flame, Nicole, who stirs old feelings and complicates matters further.
On day 26, Erica challenges Matt to prove his commitment to her by completing his vow, while Matt finds himself fending off increasingly bold enticements from his colleagues. The pressure mounts as the pool amount swells to $18,000. Faced with temptation, Matt almost succumbs but escapes a compromising situation, only to reunite with Erica for a connection that doesn't lead to sex. The tension peaks when Nicole returns with her own intentions to rekindle something with Matt, only for him to deny her, which only intensifies her desire.
As Matt nears the end of his 40 days, he grapples with uncontrollable thoughts, and an unexpected encounter with John highlights the absurdity of the situation they both find themselves in. After a chaotic culmination with Nicole, who confronts him on the final day of his vow, Matt is left in turmoil. However, determined to win back Erica, he confronts her and finally expresses their feelings with a passionate kiss. They share a night of intimacy that solidifies their relationship, breaking free from the constraints that once held him prisoner. Meanwhile, Ryan and the gang, ever the gamblers, wait in anticipation and excitement, placing new wagers on Matt's newfound freedom.
