盛烈是個盡忠職守的刑警，就算是好朋友犯了法也不會徇私的人。但他與妻子志妍的好友海真卻發生了不倫關係，使夫妻倆的關係漸漸疏遠，而依然愛著妻子的盛烈在心裡內疚地自責。

志妍是國內知名的奚琴演奏家，因漂亮的外貌和出眾的才華令其贏得了不少人的愛戴，不過孩子失踪的事情始終讓其耿耿於懷，與丈夫的關係陷入膠著狀態。走不出悲傷籠罩的她接受了精神科專家的治療，但始終無法得到精神上的解脫，決定離開韓國到美國重新生活。

在志妍的歡送會結束后，盛烈被警局召回調查一宗關於放高利貸，惡名昭著的黑幫份子趙東哲被殺的命案。礦井命案現場除了發現死者被捅數刀躺在血泊中外，盛烈頓時陷入恐慌中，因他發現遺留下來的線索像是今天早上外出的妻子。

謎團似乎越來越多，盛烈在逐步接近真相的同時也使自己走向了危機的邊緣。為了救回妻子，盛烈單槍匹馬找光哲作最後的對決。在對決中，正義戰勝了邪惡，盛烈為此受傷入院，這時皮耶羅也來到醫院向其說出了事情的謎底…

“Secret” is essentially about an estranged couple dealing with the loss of their child. The distance between them can also be traced back to the fact that Seung-yeol had an affair with Ji-yeon's best friend, and it is the reason he loses his own child in a car accident. Seung-yeol is a detective in charge of violent crimes. He finds himself in a great predicament when he spots traces of his wife, Ji-yeon at a homicide scene. He presumes that his wife is connected to the case, and does all he can to keep her from being identified as a suspect by covering up the evidence. He had his good buddy-and-colleague, Detective Choi suspended from duty for two years by reporting a pretty understandable misdemeanor and he was recently back on the job, is naturally out for revenge. Meanwhile, footage from a security camera is about to give away the fact that Ji-yeon had been at the crime scene. To complicate matters, the murder victim turns out to be none other than the brother of a powerful gang boss by the name of Jackal, who is more eager than the police to catch the killer. Seok-jun emerges as a suspect but Seung-yeol still has his doubts about his wife. Jackal comes in to give Seung-yeol a lesson to explain why Seok-jun is not the prime suspect. When an unidentified man shows up claiming to know Ji-yeon is involved and asking for money, the situation is made worse by Ji-yeon refusing to tell him any kind of truth. An anonymous caller who seems to know a big secret starts blackmailing Seung-yeol.