A murder has occurred and the police are unable to find loopholes in the alibi of the main suspect, thus facing obstacles in the investigation.

Tetsuya Ishigami (Shinichi Tsutsumi) is a reclusive, but brilliant mathematics teacher, who lives next door to Yasuko and her teenaged daughter Misato.

Ishigami is solemn and introverted, and his morning exchanges with Yasuko, from whose restaurant he buys lunch, is the brightest part of his day.

A high level battle of wits ensues as Ishigami tries to protect Yasuko by outmaneuvering and outthinking Yukawa, who faces his most clever and determined opponent yet.

Yukawa finally succeeds in cracking the case only to reveal a sad and shocking truth, that will do no good to all parties, save the police.