Seo-jun was once badly hurt by a friend's cruel lie. Since then, he has studied profiling to read the people around him, separating truth from deception. When a transfer student named Shin-yul arrives, he tries to profile her and fails. Driven by a competitive streak, he keeps trying but fails again and again. One day, returning to the classroom during PE to find his water bottle, he stumbles on Shin-yul’s diary. He takes it, telling himself it will help. At that moment, he spots someone slipping something into Shin-yul’s bag. After PE, the classroom erupts when Ye-ji’s earphones are missing. Through location tracking, they turn up in Shin-yul's bag. She is blamed. Seo-jun starts profiling again ― this time to prove her innocence.