What would you do if you found out you were a character in a comic book? And a supporting character, at that? You’d change the course of the story, of course! It tells the story of Eun Dan-O (Kim Hye Yoon), a girl who realizes she’s a supporting character in the comic “Secret” and goes against the story, thwarting the author’s plans for her character by using the storyboard that only she can see—all to win Ha-Roo’s (Rowoon) heart and change her life. Eun Dan-O is a student at a prestigious school. Her family is wealthy, but Eun Dan-O has a heart condition. Her boyfriend, Baek Kyung (Lee Jae-Wook), attends the same school. He really likes her, but she doesn’t feel the same way about him. Suddenly, Eun Dan-O experiences short-term memory loss and finds herself able to see what will happen in the near future; she is confused. Then, Jinmiche (Lee Tae Ri), from the school cafeteria, tells Eun Dan-O that she is a character in the comic "Secret," just like everyone else. Eun Dan-O discovers that her character isn’t even a main character, but rather a supporting one, and she doesn’t have much time left to live. Eun Dan-O is stunned, but decides she wants to find her true first love—not someone created by the writer. One day, she falls down the school stairs. She feels her back brush against the back of another student, who is standing among a group of boys. Her heart suddenly begins to flutter. Eun Dan-O wants to find the student who made her heart flutter. She eventually finds him (Ro Woon), and he turns out to be her classmate. He’s a character with no name, nicknamed "Number 13," and he, too, is a supporting character in the story. They approach each other, and Eun Dan-O calls him "Ha-Roo." Meanwhile, Yeo Joo Da (Naeun) is a poor but optimistic girl with a pretty face and a kind heart. She is an energetic character who always has a smile on her face despite the privileged students around her (she is the protagonist of the comic). Oh Nam Joo (Kim Young Dae) is a second-generation chaebol and the nation’s No. 1 wrestler, also known for his good looks. He has all the traits of a devilish bad boy, including a rude personality and hidden inner pain (he is the comic’s protagonist). Lee Do Hwa (Jung Geon Joo) is a friendly and charming young man. He always smiles brightly and acts playfully in front of others, but in reality, he is someone who hides his loneliness and sadness. Jin Mi Chae (Lee Tae-ri) is a mysterious student whose real name, age, nationality, and background are all unknown. Instead, he is known as "Jinmichae" (jinmichae is a banchan made from dried squid). The nickname stems from his tendency to always show up when the cafeteria menu features jinmichae. Because of his good looks, he is sometimes referred to as the unofficial fourth member of the school’s "flower trio," known as “A3.”.