Dan Oh, a high school girl who is the only child from a wealthy family, has congenital heart disease. One day, she wonders why her memory keeps disappearing. She finally realizes that she is a character in the cartoon and decides to find her real life and love. Meanwhile, a character that doesn't even have its name, Seuli High School's Student Number 13, lives like a background of the other characters, but his life slowly changes after meeting Dan Oh. They begin their mysterious adventure to find out the secret of the romance cartoon world, and they try to get their independent lives back.