Kate Mosley (Jennifer Anniston) is confident in her abilities but her advertising career seems to have hit a snag. When she asks her boss Mr. Mercer (Kevin Dunn) why she keeps getting passed up for promotions he tells her it is because she lives a single, carefree lifestyle and could leave Mercer Advertising at any time. Kate and her friend and co-worker Darcy O'Neil (Illeana Douglas) concoct a plan that involves a fake fiance for Kate. They choose Nick (Jay Mohr), a freelance videographer from Massachusetts that they met at a recent friend's wedding. Mr. Mercer, excited that Kate is living what he considers a stable lifestyle, is anxious to meet her fiance and arranges a dinner.
Kate's imaginary relationship with Nick also stirs thoughts of romance with co-worker Sam Mayfair (Kevin Bacon). Kate was always attracted to Sam before but he was never interested because Kate was unattached and available. Now that Kate is supposedly engaged to be married and, in theory, unavailable, Sam becomes very interested in her.
Nick agrees to come to New York and go to dinner with Kate and her co-workers. The two rehearse what Nick should say at the dinner and plan a dramatic break-up of their relationship in front of Mr. Mercer and his wife. Kate accuses Nick of cheating on her at the dinner. Nick, playing along, admits to having a mistress and justifies his actions by saying that Kate also has a mistress. Her mistress, Nick says for everyone at the dinner to hear, is her love of Mercer Advertising.
The next day Mr. Mercer offers Kate the promotion she always wanted. Kate accepts but grows increasingly uncomfortable because she knows she lied to get the promotion. Still feeling a void in her life Kate seeks out Nick again. She shows up at a wedding that Nick is filming and he agrees to ask her to the reception as his guest and to begin a real relationship.