In 2004, the Chinese-Korean gang known as the "Heilong Gang" from Harbin, Heilongjiang Province, China, suffered a major blow due to a severe crackdown by Chinese authorities. Following this, gang member Zhang Chen (also known as "Zhang Qian," portrayed by Yoon Kye-sang) fled to South Korea along with two associates, Wei Chengluo (played by Jin Seon-kyu) and Yang Tai (played by Kim Sung-kyu). 

The three initially began operating in the Yanbian Street area of Garibong-dong, Guro District, Seoul. Zhang Chen, who referred to himself as "Harbin Zhang Chen," was known for carrying weapons such as knives and axes. His methods were notoriously ruthless, ranging from inflicting severe injuries to causing fatalities. His actions created an atmosphere of fear and lawlessness in the area.