The film is adapted from the masterpiece "The Devotion of Suspect X" by Japanese mystery writer Keigo Higashino. The original name was "Perfect Love", but it was later changed to "Suspect X" which is close to the original. The only pleasure of the taciturn genius mathematician Jin Shigu (played by Ryu Seung-fan) every day is to go to a fixed bento shop to buy lunch, just to take a look at his neighbor Bai Huashan (played by Li Yaoyuan) who works in the bento shop. Huashan and her niece Yuner depend on each other and live a peaceful life, but there is an ex-husband Jin Zhemin who always comes to trouble. Finally one day, Huashan couldn't bear the abuse and finally killed her ex-husband who came to entangle. After Shigu realized that Huashan killed someone in self-defense, he took the initiative to ask him to take care of the aftermath. For this crime, he designed a perfect alibi to ensure that Huashan and Yuner would not be implicated. Officer Zheng (played by Zhao Zhenxiong), who is in charge of investigating the case, is a good friend of Shigu. He has no doubts about Huashan's identity as the murderer. Although there is no conclusive evidence, he still stubbornly investigates. What kind of hiding method did Shigu use? Will he finally sacrifice himself for love?