After being unceremoniously dumped by her boyfriend Charlie, Lauren finds herself in a tough spot, unable to afford her own place. Fortunately, her old friend Jesse, played by Justin Long, informs her of a charming apartment near Gramercy Park. However, he omits the crucial detail that his other friend, Katie, portrayed by Ari Graynor, is already living there. Their history is rocky—during their college days, Lauren had to drive a drunken Katie home from a party, which ended in an unfortunate incident involving a slushie container and Lauren's car. Despite their evident animosity, they decide to become roommates out of necessity.
Things take an unexpected turn when Lauren overhears strange noises from Katie's bedroom one night. Fearing for her safety, she barges in only to discover Katie engaged in an explicit phone call. Katie reveals the surprising truth: her job as a writer is not enough to sustain her in New York City, so she has resorted to working as a phone sex operator. While initially horrified, Lauren attempts to help Katie strategize ways to increase her income.
Weeks later, Lauren loses her job at a publishing company due to her boss's retirement. Prompted by their earlier conversations, she begins to assist Katie in establishing their own phone sex business, naming it "1-900-mmmhmmm." This venture proves to be financially successful; within a short span, they rack up $12,000, with Lauren earning a significant cut, especially after getting involved in the phone operations herself. As they navigate this unconventional business, they forge a strong friendship.
Throughout the film, Katie develops a connection with a repeat customer named Sean. Their conversations transition from mere phone sex to meaningful discussions, leading them to agree on a face-to-face meeting—much to the chagrin of Lauren and Jesse, who fear for Katie's safety. Contrary to their worries, Katie and Sean hit it off, but confessions of Katie being a virgin stir up her insecurities, adding complexity to her budding relationship.
In an unexpected turn, Lauren receives a call for a job interview at a prestigious publishing company that had rejected her previously. Encouraged by Katie, she initially considers blowing it off but surprises everyone by openly discussing her phone sex line experience during the interview. The company's interest in her "business savvy" prompts Lauren to accept the position, which infuriates Katie, leading to dramatic revelations about their business to Lauren's disapproving parents. This revelation strains their friendship, resulting in Lauren moving out.
Not long after, Lauren's ex-boyfriend, Charlie, returns from Italy, professing his regrets over their breakup. He reveals an affair with an adventurous Italian woman but insists he needs someone mundane for his future. Lauren bravely defies him, choosing to reaffirm her own worth and ending any possibility of reconciliation.
Meanwhile, Katie and Sean finally make the leap into intimacy while sharing a phone call from their beds, discussing their feelings. Afterward, Katie feels an overwhelming desire to share her happiness with Lauren, underscoring the affection they developed throughout their turbulent living arrangement. They both instinctively reach for their phones at the same moment, resulting in a whimsical conversation filled with awkward but endearing innuendo. Ultimately, they reunite on the street, embracing in a heartfelt hug that signifies their friendship's resilience through all challenges.
