A third female corpse appears in an abandoned factory, with strange knots tied around her wrists. Criminal investigation captain Cheng Yu was forced to seek help from a top criminal psychologist, but he encountered the most painful accident in his life - the person who assisted in solving the case turned out to be his lover Lin Muqing, who broke up without saying goodbye three years ago.
Soulmates in college broke up completely due to a fatal misunderstanding during an undercover mission. Now we meet again, she is a profiler who calmly analyzes the murderer's psychology, and he is a cold-blooded policeman who only believes in evidence. When Lin Muqing pointed out that the knot hid the surgical code, Cheng Yu discovered the truth about the serial murder of jurors from the perspective of criminal investigation, and the two had to work together to hunt down the murderer.
However, old wounds have not healed, and new cases arise. The Knot Killer suddenly escalates his actions. Lin Muqing recognizes the identity of the murderer in the death threats - the son of a nurse who died on the job in a medical accident case. The revenge list targets the person who was sentenced that year. In a life-or-death confrontation on a rainy night on the rooftop, Cheng Yu reveals a hidden secret to save her: he disappeared three years ago to trace the larger pharmaceutical conspiracy behind this case.
When the mastermind behind the scenes pointed the poisonous blade at Lin Muqing's relatives, she used her body as bait to get into the trap. Cheng Yu breaks through the fire to save his lover, but falls into a darker net: the knot's poison spreads to his arm, and the real culprit is Lin Muqing's mentor, Professor Chen, a master of criminal psychology.
The former mentor arranged a murder at the graduation ceremony of the medical school, and the knot changed from a criminal symbol to a torture tool to control people's hearts. In order to end this crazy experiment in the name of justice, Lin Muqing used the psychology he taught himself to counterattack, and Cheng Yu used his flesh and blood to block the blade attacking his sister.
