[2] Adapted from Keigo Higashino's novel The Devotion of Suspect X, it centers around a mild-mannered mathematics teacher (Ryoo Seung-bum) who plans the perfect alibi for the woman he secretly loves (Lee Yo-won) when she unexpectedly murders her abusive ex-husband.

Kim Seok-go showed a lot of promise as a brilliant mathematician when he was in school, resolutely focused on his studies rather than on friends throughout his childhood.

Seok-go is solemn and introverted, and his morning exchanges with Baek Hwa-sun, the cafe employee he buys lunch from, are the brightest part of his day.

Seok-go overhears the fight from his house next door and decides to cover up the killing and protect her from the police.

However, the detective in charge, Jo Min-beom, believes that Hwa-sun is guilty and follows his intuition despite the lack of evidence.