Mary (Moon Geun-Young) is a young woman with the energy of the sun. She looks like her late mother, but her hot temper comes straight from her deadbeat father (Park Sang-Myeon). Her father always makes trouble as a failed businessman, but Mary loves him dearly, despite his faults. Because Mary can't afford to pay her college tuition fees, she temporarily stops attending school. Without any special skills, Mary works odd jobs just to put food on the table and to keep from getting too far behind on the rent. One day, Mary's father proposes that Mary be wed to a man named Jung-In to pay off their debts. When Mary hears of her father's plan she devise a plan of her own. Mary takes fake wedding pictures with her new friend Mu-Gyul (Jang Geun-Suk), whom she just met over the last couple of days. She sends the pictures to her father's cellphone with the message that this is the man she loves and they've gotten married. Unfortunately for Mary, her deadbeat father has already registered a marriage certificate on her behalf with Jung-In. Torn between having her father prosecuted for fraud or becoming a 24-year-old divorcee, Mary is somewhat relieved when Jung-In proposes that Marry spend 100 days of marriage with both men and decide with whom she wants to live.
Mu-Gyul (Jang Keun-Suk) is the lead singer and guitarist for an indie rock band. He is chic, cool, talented, and loyal, living a carefree bohemian lifestyle. He doesn't talk much and only wants to make music. Mu-Gyul doesn't care about success. He also has trust issues from his mother, who's every bit as well-meaning, likable, childish, & unreliable as Mary's father. Because of this, Mu-Gyul has a constant rotation of women in his life, not believing in any of them enough to consider sticking around. Besides that, he's so unfamiliar with fidelity that the idea of a lasting relationship seems alien, strange, and a bit scary to him. When it appears Mu-Gyul will never get married a once-in-a-lifetime opportunity comes about. He meets Mary, money becomes involved, and a 100 days of marriage ensues. During this time, Mu-Gyul, not knowing the meaning of family, surprisingly falls in love with Mary who is combative, loyal, and spunky. Mu-Gyul finds himself becoming a rival of Jung-In for Mary's love.
Jung-In (Kim Jae-Wook) is the president of music company JI Entertainment producing a music drama. Jung-In was born into a wealthy family and has never lacked money. Jung-In is also good looking and has good manners. Because of this, Jung-In appears to be highly eligible bachelor for marriage. However, Jung-In also has a cold side - he doesn't like becoming too close to people. His weakest point is that he can never say no to his father. He enters into a 100 day fake marriage with Mary and soon falls in love with Mary. Jung-In also comes across Mu-Gyul who will help fulfill Jung-In's dream in the music world. Mu-Gyul will also become a rival for Jung-In over Mary. Then Seo-Jun enters the picture.
Seo-Jun (Kim Hyo-Jin) is a sexy, sophisticated woman. She also comes from a rich family. Seo-Jun works as an actress, preferring to make movies rather than dramas. Seo-Jun broke up with Mu-Gyul about a year ago, but still attends his performances without his knowledge. She also has a care-free spirit similar to Mu-Gyul's, but uses this to cover up how emotionally fragile she is. Seo-Jun reunites with Mu-Gyul and convinces him to sign a contract for a drama produced by Jung-In. Seo-Jun starts to develop feelings for Jung-In in a bizarre love square.
Ultimately, "Marry Me Mary!" is the story of three young people trying to salvage the wrecks made of their lives by the shameful ways their parents have lived. Mu-Gyul's mom is flighty and cares more about her latest man than her son. Mary's dad is semi-competent at best and easily conned. Jung-In's dad, while a successful businessman, only wants Mary to wed his son to satisfy the sick obsession he's had with Mary's mom, an obsession that ultimately led to his divorce of Jung-In's mom. Can the three grown children of such dysfunctional parents find true happiness?