Kim Jin Seo (Kim Hye Soo) and Mo Yoon Hee (Hwang Shin Hye) are high school classmates and rivals whose different family backgrounds shaped their characters and their paths towards love and success in adulthood. Jin Seo develops a warm and cheerful personality as well as superior intellectual ability, while Yoon Hee becomes a beautiful woman with fatal charm. Both women love Lee Sang Hyun (Shin Sung Woo), an extremely kind but indecisive man who used to date Yoon Hee but later married Jin Seo. When Yoon Hee's husband, Sung Eun Pil (Kim Gab Soo), dies under suspicious circumstances, Jin Seo finds herself becoming involved in the investigation. The process is complicated when the young homicide detective Kang Shin Woo (Lee Sang Yoon) becomes romantically interested in Jin Seo.