The young woman in question is named Thea Clayhill, played by Lindsay Lohan. She works as an assistant at a publishing company and is constantly struggling to keep up with her job and her bills. When she mistakenly thinks she's about to be fired, she comes up with the idea to fake a pregnancy in order to keep her job.
Thea's plan works better than she could have imagined - her co-workers are suddenly much more accommodating and her boss, Jerry (played by Chris Parnell) even offers her a promotion. However, as Thea's "pregnancy" progresses, she finds it harder and harder to keep up the lie. She enlists the help of a friend, Lisa (played by Cheryl Hines), who helps her create a fake baby bump and teaches her how to act like a pregnant woman.
As the months go on, Thea becomes increasingly wrapped up in her lie, even going so far as to fake labor pains in order to get out of attending a work event. Her love interest, Nick (played by Luke Kirby), becomes suspicious of her pregnancy and starts to piece together the truth.
When the truth is finally revealed, Thea is fired from her job and finds herself struggling once again. However, she's learned valuable lessons about honesty and hard work. The film ends with Thea starting a new job as a writer and Nick coming to see her at her new workplace.