One day, Yejin's best friend, Haeri, suspects her of stealing a secret diary.

In the summer of 2008, Yejin and Haeri are the best friends in their class. However, their bond begins to crack when Haeri starts keeping a secret diary with students from another class. As Yejin discovers the secrets written in the diary, she grows uneasy and when the diary suddenly goes missing, Haeri suspects Yejin, the person closest to her. Once inseparable, the two now hurt each other while trying to protect their own secrets.

Small misunderstandings leave deep scars, and secrets once shared can easily break a bond. A Secret Diary explores memory and relationships through the doubts and conflicts between two best friends over a missing diary. As Yejin and Haeri, once inseparable, hurt each other in ways only close friends can, the film questions the fragility of the relationships and memories we believe in. What were we truly searching for?