Shi Hong is a brilliant mathematics teacher who is a recluse but likes the active company of his neighbor, divorcee Chen Jing and her young daughter through the thin walls. When Fu Jian, Chen Jing's abusive ex-husband, suddenly shows up trying to extort money from Chen Jing, threatening both her and her daughter, the situation spirals out of control and Fu Jian is killed. Shi Hong offers to help Chen Jing and her daughter cover up the crime, but when his former classmate and police consultant, Tang Chuan, becomes involved in investigating the murder, Shi Hong must betray his friend's trust and involve himself in a dangerous cat-and-mouse game that reveals the depths of his feelings for Chen Jing and her daughter. Adapted from the Japanese novel "The Devotion of Suspect X" (容疑者Xの献身, Yōgisha Ekkusu no Kenshin) by Higashino Keigo 东野圭吾.