Little Seung Jae stays up all night and writes letters to So Hee, pretending to be her father on Mars. Leaving Seung Jae's innocent heart behind, So Hee leaves for a big city. Both grown up to be adults, Seung Jae is a postman who writes letters to So Hee's grandma--the letters that never came from So Hee. So Hee's heart hardens after hardships in life and she comes back to her hometown and Seung Jae's heart swells up anew at seeing her again.