"Marriage Blue" follows four couples as they prepare for their weddings.

Tae-Kyu (Kim Kang-Woo) is a former professional baseball player and now a coach for a minor league team. His girlfriend Joo-Young (Kim Hyo-Jin) runs a successful urology clinic. They seem happy and are set to get married soon. Tae-Kyu then learns that Joo-Young is a divorcee. Tae-Kyu feels betrayed and also jealousy.

So-Mi (Lee Yeon-Hee) is a nail artist. Her fiancé is Chef Won-Cheol (TaecYeon). They have been together for 7 years and their wedding date is fast approaching. So-Mi though feels something missing in her life. She wants to feel pure love, but Won-Cheol thinks of their marriage more of as a living arrangement. Several days prior to their wedding, So-Mi travels to Jeju Island for a nail art competition. There, she meets tour guide and webcomic writer (Joo Ji-Hoon).  

Gun-Ho (Ma Dong-Seok) runs a flower shop and is about to marry Vika (Guzal Tursunova) who hails from Uzbekistan. Suddenly, Gun-Ho is afflicted with impotency. Gun-Ho goes to see urologist Joo-Young. She tells him that there is nothing wrong with him physically and his impotency is caused by stress. Vika is younger than him and very beautiful. Gun-Ho begins to wonder if Vika is only marrying him to attain Korean citizenship.

Dae-Bok (Lee Hee-Joon) works for Joo-Young at her urology clinic. He is dating wedding planner Yi-Ra (Koh Joon-Hee). One day, during a baseball game, Yi-Ra tells him that she is pregnant. Dae-Bok immediately proposes. While they prepare for their wedding, they begin to argue a lot. Yi-Ra doesn't seem happy and Dae-Bok wants to know why.