Ma Shan (Jiang Wen) is an ordinary policeman in a small border town in the southwest. One night, he was drunk at his sister's wedding banquet and was sent home. When he woke up the next morning, he found that the gun he never left was missing. When he realized that the thief could use the three bullets in the gun to kill people, Ma Shan couldn't help but shudder. Everyone who attended his sister's wedding that night became his suspect. After tracking down the key suspects, Liu Jieba, who sold "mutton noodles", Zhou Xiaogang, who sent him home, and Li Xiaomeng (Ning Jing), an old lover who suddenly appeared at Zhou Xiaogang's house, Ma Shan still couldn't figure out any clues. His suspicion of Li Xiaomeng was mixed with extremely complicated emotions, which led his wife to suspect that he had changed his mind. At this time, Li Xiaomeng was shot dead at Zhou Xiaogang's house, and Ma Shan was detained as a suspect. After some calm thinking, he finally straightened out the mess and began his journey to find the gun.