In that snowy season

In an ancient northern city, the four best brothers Hong Jian, Hong Feng, Liming and Zhang Xiaotong grew up together. Many years later, Hong Jian became the captain of the serious crime team. When he was investigating the "underground human trafficking network" of the smuggler group, a series of murders occurred in the city. The dead were all young women, and a mysterious fire butterfly was left on the bodies. At this time, Hong Feng, the president of his brother's bank, was involved in a murder case. The person he was suspected of killing was the woman Hong Jian once deeply loved, Hong Feng's current wife Ye Yu. Hong Jian's two other good brothers, one became a confidant of the human-snake group, and the other became a ruthless thug of the underground gang. A serial killer killed a key insider of the human smuggling group. Based on the clues left by the case, Hong Jian gradually found out: every time the human smuggling group imported drugs from abroad, it also smuggled young women out of the country; and the next woman to be trafficked out of the country was Zhang Xiaotong's beloved woman Tingting. After hearing the news, Zhang Xiaotong blamed everything on Liming, a close friend of the smuggling group, and turned against him to vent his anger. Little did he know that Liming was an undercover agent carefully arranged by Hong Jian for three years. At the dawn of a life-and-death crisis, Hong Feng suffers from a heart attack. Hong Jian discovers that there is a huge hidden secret behind his brother's death... All of this is complicated and confusing, and the thrilling investigation is closely intertwined with family and friendship.