At the scene of a series of murders in Binjiang City, a cross mark that had been hidden for ten years quietly reappeared, opening the gap in the unsolved case that had overturned the fate of two generations of criminal policemen.
Shen Yi, the leader of the serious crime team who is calm and cold-hearted and believes in the supremacy of evidence, has a father who was a criminal police officer who died in the "119 serial murders of public officials" ten years ago.
The two people, who have conflicting views and each have their own secrets, are forced to work together because of a murder in a secret room. One is digging for traces at the crime scene, and the other is tracing the outline of the murderer from the abyss of human nature. From the tit-for-tat collision of ideals to the mutual protection at the critical moment of life and death, they peel off the cocoons from six unsolved cases in the campus, including the campus corpse burning, the abandoned hospital case, and the massacre of the family in the urban village. They discover that each murder case is paving the way for the shocking secrets ten years ago.
The ghosts in the city bureau are secretly obstructing, and the black interest chain of the real murderer is intricate. Shen Yi and Lu Xun are pulled between suspicion and trust, and they stick to the law and obsession. They not only want to solve the mystery of the serial murders and uncover the former high-ranking police officers who controlled everything behind the scenes, but they also want to clear the grievances of their fathers and comfort the souls of the dead.
When all the lies were exposed and all the evidence surfaced, the pair of criminal police partners who had gone through the hail of bullets side by side finally turned their life-and-death comradeship into a two-way sincerity when justice was settled.
The old case has been cleared and the deep traces can be traced. They are still the sharpest criminal investigation blades in Binjiang City, and they are each other's only place of peace of mind for the rest of their lives.
Purpose: Hiding unsolved cases with deep traces and searching for the real culprit