Headstrong and resourceful, Shin Na-ra once dreamed of becoming a career woman, but she spent the best years of her life supporting Kang Seung-woo, her boyfriend of seven years. Now he is a successful accountant and she is unemployed and rapidly approaching thirty. Na-ra hopes to marry Seung-woo, but after he returns from a business trip, he unexpectedly breaks up with her on their anniversary, telling her that not only did he have an affair, but he fell in love with the other woman, a cosmetics manager named Cha Mi-rae.
To drown her sorrows, Na-ra drinks a large amount of alcohol and unknowingly drops the ring Seung-woo once gave her into a shot glass of soju. A stranger, Na In-jae, drinks the shot without seeing the ring and both of them end up in the hospital.
Later, Na-ra decides to confront the woman who stole her boyfriend, but while at Mi-rae's cosmetics company, a job ad catches her eye. With nothing to lose, Na-ra fakes being her 20-year-old sister Na-kyung, a high school graduate, and starts working as a sales clerk at the company. Just her luck, Na-ra's co-worker is In-jae, who turns out to also be in love with Mi-rae. The two constantly clash at work, with Na-ra finding In-jae immature and spoiled. But what she doesn't know is that In-jae is actually the son of the company's vice president who was forced by his mother to learn the ropes by starting from the bottom.