Hongyu accused of familicide, is in hiding. His twin, Guan, a nyctophobic, is an ace detective. They want the police file on the familicide, to prove Hongyu innocent. Guan probes a multiple murders case, and Hongyu replaces him at nights.
During Hongyu's night foray, they find body parts of 2 more persons. Guan makes a deal with Captain Zhou to help him solve cases in return for being able to see his brother's familicide file. The current crime points to a murder spree.
Hongyu outmaneuvers the killer. Police arrives and Guan is considered the victim. Guan puts in a recommendation for rookie cop Zhao Qian, who had been secretly ordered to tail him.
Guan had put away Qi in prison for gang related crimes. Upon release, Qi spots Houngyu, the twin, quarrels with him and is knocked down. Later, he is knifed and murdered. Guan worries because DNA under Qi's fingernails may match Hongyu's.
Houngyu ventures out in the day and finds that Yaoji deals in drugs. This tip helps Guan nail Yaoji and his boys. The police are puzzled how Guan knew about the drug dealing. Qi's murderer is still at large, although Ah Man is suspected.
Police believe Wang Hui killed Qi. There are anxious moments when Houngyu switches with Guan at night, during the police raid. Zhou poses as a prostitute to trap Wang. Wang tells the police that Yaoji killed Qi, but Yaoji implicates Wang.
Guan gets a lead from Houngyu to Yaoji's drug stash. He suspects if Houngyu was into drugs earlier. Liu takes Zhou to the scene of the familicide, which Houngyu is accused of. There, they discover a dual murder in a car parked outside.
A double murder of a celebrity and a girl creates a stir. There is a furor when the press discovers that Guan is still with the police and Guan now has 1 week to solve the case. Sensing a serial killer's work, outside police join in.
The press leaks the MO of the serial killer. Another double murder happens. Although a copycat killing, the press is told that it is a fresh serial kill. Guan learns that Houngyu has been implicated in the familicide by false testimony.
The police analyst surmises, with logic, that this serial killer's first kill has not been unearthed yet. That first kill can give vital clues about the serial killer. So, the police start digging again and find a 5-year-old murder.
After much work and deduction, the police have a suspect who is a Forensics expert and a germaphobe. But, they lack the evidence to nail him. Gaun gets a breakthrough in the video of the crime. His position with the police is secure now.
A car crash and 2 murders happen in quick succession. Serial killer Zhige escapes. The hotel room key, evidence against his wife Ying, is missing. Police fear Gaun is Zhige's target. The brothers have a falling out and Hongyu leaves.
Zhige diverts the police on a wild goose chase and reaches the police station in search of Gaun and the familicide file. Liu is taken hostage. A deeper, sinister plot is suspected.
Guan suspects that there are people behind An Teng, who are involved in the familicide too. Yanan is brought into the secret about Hongyu. Zhou enters Guan's home, looking for clues to nab Hongyu.
Hongyu tangles with Guo in a bar, who drops his cap. Hongyu spends the night with Ren Di, the bar singer. The cap gets casually passed on to the singer's brother. Thugs wrongly kidnap this cap wearing brother. Yanan is flustered about Ren.
The victim's finger is cut by the kidnappers. The police search continues. Ren drugs and kidnaps Guo to force his father to pay the ransom to free her brother. Liu feels that Hongyu, Ah Teng and Ye Fangzhou, a disgraced cop, are connected.
One of the kidnappers escapes after being caught. Later, his body is found. The kidnap victim dies in hospital due to extrinsic asthma led hypoxia. Ren, to release Guo, wants the 2 kidnappers. Guo's dad offers a reward for the kidnappers.
Hongyu trades Hu Qiang for Guo Peng, with Ren Di. He convinces her not to harm both. Hongyu, disguised as Guan, goes to Jiangzhou to investigate Hu's burglary. Guan, as Hongyu, investigates at home, is spotted and chased by the police.
The Jiangzhou police draw a black about Hu's burglary 2 years ago. Hongyu and Shutong visit the burgled home. The owner, Dong Qian, is a professor. He later disappears, leading to a search. Guan shakes off Captain Zhou after a long chase.
Hongyu and Shutong hunt for Don Qian in Hongyang. They search in a plantation and find 5 skeletal remains of young men. Later, they find Qian hanging from a tree. But, Hongyu feels that Qian did not commit suicide. Guan is in Jingang.
Hongyu and Shutong re-enact the hanging and finds inconsistencies. The culprit is finally identified as Feng, the employee of the building management company. He is traced, and the bottle of 1961 Lafite red wine is also recovered from him.
Guan agrees to go undercover to infiltrate an arms-trafficking ring, for which Zhou feigns, in public view, a bitter quarrel with Guan. However, Hongyu gets the initial breakthrough in this task, when he reaches the No. 2 guy in the ring.
Hongyu meets Jin Shan, the arms dealer. But, Lin Jaiyin, the undercover lady cop who is now one of them, wants him dead. Before dealing, Hongyu wants Lin killed. Gaun disagrees, goes and replaces Hongyu and retracts this condition.
Boss Meng scolds Jin Shan, but Lin Jaiyin suddenly kills him. Jin is now boss. By this, Lin gains more favor with him. She also exposes another undercover agent, the buyer Jie. Guan becomes a trusted supplier. The subterfuge continues.
Jin Shen fights Lin and Zhou. Jin is fatally wounded. Lin survives, but escapes. Cindy is shot and blows up fuel barrels. She is charred and police assume that Lin died and Cindy escaped. Gaun pursues a lead to the gun buyer in Changchun.
Guan and Liu reach Chungchun separately in search of Qiao Sen. Liu fares better. Guan is trapped in the snow in a badly hit car, but Hongyu reaches and they survive an overnight trek in snow. Zhou disapproves Liu's journey to search Qiao.
Shutong warms up to her father, Liu. At night, Zhou sees through Hongyu's disguise and confronts him. Ye Fangzhou is pressured to defuse the growing danger from the police, who had reached Changchun to seek out Qiao. He plans to kill Gaun.
Liu suddenly collapses and dies in the police station. Police find arsenic and mercury poisoning in his milk tea, which Shutong had brought for him. Zhou Xun is arrested as a suspect. CCTV footage is inadequate in the investigation.
Hongyu has grave doubts that Gaun is complicit in the familicide. Shutong investigates her father's murder, leading to a sudden face-off with Ye Fangzhou. Just then Ye is suddenly killed by someone unknown. Han Bin is quietly helping Gaun.
