Seok Ho was a child genius but now lives an average life as a high school math teacher. When his new neighbor, Hwa Sun, moves into the apartment next door, she becomes his new obsession. One day, Suk Go witnesses Hwa Sun murder her ex-husband during a violent encounter. He puts his math skills to the test as he calculates the perfect alibi for her crime. Thanks to Suk Go’s impeccably crafted crime, Hwa Sun even passes the polygraph test and the police have no choice but to let her go. However, the detective assigned to the case, Min Sun, detects something strange and is unable to drop Hwa Sun as his prime suspect.