A story that takes less than 24 hours. In order to find his missing girlfriend, Lai Fu (played by Daniel Wu) immigrated from the mainland to Hong Kong and found a job in Mong Kok. He went astray and became a killer, and was involved in the bloody storm of the underworld. The one who helped him was the treacherous Lao Liu, who was mixed in both the black and white worlds and was an informant for the police. Lai Fu embarked on a desperate journey. During this period, he met the prostitute Dan (played by Cecilia Cheung). The similar life experience made the two of them feel like old friends. Dan was determined to help Lai Fu find his girlfriend and change herself at the same time. The police tried to arrest Lai Fu many times but failed. Seeing that the grudges in the underworld were gradually subsiding, they were about to give up. Lai Fu and Dan seemed to be able to take a breath, but things took an unexpected turn. Mong Kok was always undercurrents under the night.