A police detective who loses everything to a criminal organization seeks payback when he gains special abilities through biotechnology.

Son Ji Eun moves to a small city due to her husband's new job as a civil servant. She works part-time at a mart. Son Ji Eun is quiet and sincere. Her life in the small city becomes boring. Son Ji Eun's life is shaken by Yoon Jung Woo. Yoon Jung Woo is a biology teacher at a local school.

To combat rising violent crimes, the Police Chief asks Detective Oh Goo Tak to form a team consisting of criminals. He is currently suspended from the police force for using excessive force. He then gathers team members: Park Woong Chul who is a gangster, Lee Jung Moon who is the youngest serial killer with extraordinary intelligence and Jung Tae Soo who is a contract killer. Police Inspector Yoo Mi Young also joins the team and she tries to have these guys work as a team by dealing with them rationally and sometimes emotionally.

Three years ago on a raining day, Ban-suk goes out to find her husband, Wang-bae who is having an affair. That day, her son is killed in a hit-and-run and Dong-wu finds out his girlfriend, Mi-su has left him and their son. Three years later, Ban-suk gets out of a rest home and plans to seek revenge when her husband asks for a divorce. She decides to move to the same apartment complex where he and Hong-hwa are living together. Next door, Dong-wu, a part-time trainer at the gym Ban-suk is working at, is living there. Ban-suk sees Hong-hwa flirting with Dong-wu at the gym and is disappointed that Dong-wu goes along with that. She begins to see him as a playboy that her father and her husband were and hates him. Since then, they have conflicts about everything. Meanwhile, Dong-wu’s ex-girlfriend and his son’s birth mother, Mi-su gets in an accident at a club and gets help from Ban-suk’s brother, Jong-hyeok. One day, Ban-suk meets Dong-wu’s son and begins to understand who Dong-wu really is. After they realize they both have grudges against Ban-suk’s husband, Wang-bae, they become closer friends. When Dong-wu’s son treats her like his mother, Ban-suk is reminded of her dead son. Dong-wu, who is helping Ban-suk overcome her difficult past, falls in love with her without realizing that he’s the one who caused her pain three years ago.

Her lover has never taken a dangerous risk in his life. She has been for seven years and is going to get married next month. One day she meets a handsome stranger, who asked her to spend one day with him. It might be a mistake to accept his proposal, but over the next day she will experience a level of excitement and intimacy, she has never experienced before, and this could change her life all of sudden.