The Symbol of Sin

Psychologist Qin Lan is recruited by the police to assist with a series of ritualistic murders. Each victim was strangled after being injected with a sedative, their bodies carefully arranged in a posture of repentance, with mysterious symbols carved into them.

When Qin Lan discovers that the first victim was a high school classmate she had lost contact with years ago, and that her own contact information was saved on the victim's phone, she realizes the case is secretly connected to her. As the investigation deepens, more victims emerge. Without exception, they are all linked to the "Penitent Psychological Experiment," led by Qin Lan's father twenty years ago.

The experiment records show that twelve test subjects were forced to confront their deepest sins. Now, one after another, they are dying in poses of "repentance." Even more unsettling, every crime scene contains clues pointing to Qin Lan, as if the killer is leading her to uncover a truth that was deliberately buried.

When Qin Lan finds a photo of her missing father in the latest victim's hand, along with the blood-written words "The fourth is the traitor" on a wooden plaque, she has no choice but to wonder: the father she has searched for for twenty years may be the one who started it all...