In the late autumn of 1932, a series of bizarre murders occurred in the French Concession in Shanghai. All the deceased had no external injuries and died peacefully. There was only a burnt black sandalwood sign left in the palm with the word "Lock" written on it. The murderer was nicknamed "Yan Suo", which shocked Shanghai and made people panic.
The cynical but thoughtful police detective Lu Yanci was ordered to investigate. At the scene of the murder, he reunited with an old friend he had left without saying goodbye three years ago - Shen Zhiyi, a taciturn female forensic doctor who had returned from studying abroad. She read the deadly secret poison and acupuncture techniques from the corpse, leading the clues to an old case of burning down a library in Jiangnan for 20 years.
As the investigation deepened, Lu Yanci was shocked to realize that all the deceased were related to the massacre that year, and the calm and professional female forensic doctor beside him was a survivor of the massacre and one of the masterminds of the serial murders.
Revenge, truth, and conspiracy are entangled. She uses the forensic doctor as a blade and the murder case as a sacrifice, and just wants to pay with blood; he uses the detective as a protector and the bottom line as a rope, not wanting to see her fall into the abyss.
When the bigger shadow behind the scenes surfaced, she held the poisonous needle while he stood in front of her.
Should we let hatred burn everything, or should we grasp the glimmer of light at the end of darkness?
On the day when the old case was revealed and the fog cleared, in the rainy night in Shanghai, only a pair of people were left walking side by side on the edge of light and darkness.
