Se-ho Jung

Shin Na Ra, a woman who is almost thirty welcomes her boyfriend of seven years back from his business trip. During their dinner, he tells her that he not only had an affair but fell in love with another woman. Thus their relationship is no more. To make herself feel better, Na Ra drinks a large amount of alcohol and unknowingly dropped the ring her ex-boyfriend gave her into a random stranger's cup. (mydramalist.com)

M is the nickname of the soul of an aborted child that has the purpose to seek revenge on the people who would not allow him to be born. M entered into the body of the young girl, Park Ma Ri, and from this moment her life has changed and she cannot control herself anymore.