A train ride brings two unlikely souls together as Bae, a committed partner of a serious 10-year relationship, sees her world turned upside down by the charming playboy Kim. Seated together on a business trip from Seoul to Busan, the two find themselves on an adventure to remember.

A woman acts as a letter writing proxy and a man sells 'boyfriend' services. Fate brings them together and learn they each have wounded hearts. As they see each other, they look back on themselves...