Based on the internet novel of the same title and written by Guiyeoni, Do Re Mi Fa So La Ti Do is a touching love story with bittersweet elements. Jung-won wears a funny dragon suit, while working at the amusement park, and pours soda on Eun-gyu, because of his taunts. A short while later, Jung-won finds out that Eun-gyu has moved into the house next to hers. Eun-gyu now threatens to tell Jung-won's parents that she works part-time at a local mart. To keep Eun-gyu quiet, Jung-won agrees to carry his guitar for a week. Jung-won then starts to develop feelings for Eun-gyu, who is the lead singer of a band called Do Re Mi Fa So La Ti Do. Eun-gyu also starts to develop similar feelings for Jung-won. The two soon become a couple. With an odd twist of fate, Jung-won went to the band's rehearsal room and meets a familiar person from her past, a boy named Hee-won. Their relationship makes everything more complicated. For 10 years, Jung-won and Hee-won were close friends. One day, Jung-won witnessed a hit-and-run and reported the license plate to the police but the driver turned out to be Hee-won's father. Because of that, Hee-won's family broke apart and they stopped talking to each other completely after Hee-won's father was arrested. Hee-won, seeking revenge at Jun-won, told his "friends" to beat her, rendering her crippled. Jung-won is now surprised to learn that her ex-close friend is now best friends with her boyfriend. Hee-won admits that he still likes his best friend despite what she did and he wants her back. Hee-won makes her choose between him and Eun-gyu by threatening suicide. Jung-won chooses Hee-won, leaving Eun-gyu devastated. He gives her one last chance to come back to him but she breaks his heart again and chooses Hee-won. A few months later, Jung-won and Hee-won learn that Eun-gyu was in a car accident and was diagnosed with temporary amnesia. Hee-won lets Jung-won go and she spends all her time trying to get Eun-gyu to remember. The last scene is an re-enactment of the band's last show, the one in which Eun-gyu gave her a last chance. The movie ends with him remembering everything and begging Jun-won not to leave him again. She promises she never will. As the final credits roll we see how all of their lives improved.
