In order to make the outdated gang leader Dai Geda (played by Lu Haipeng) a tainted witness for the police and testify against the crimes of the gang leader Wu Erqi (played by Wang Qing), Police Superintendent Cao (played by Cao Dahua) designed to force four private detectives known as lucky stars, including Rhinoplasty (played by Feng Cuifan), Dashengdi (played by Wu Yaohan), Huatabing (played by Miao Qiaowei), and Luo Han Guo (played by Zeng Zhiwei), to go undercover. Cao also sent his daughter Banana (played by Carina Lau) to apply for a maid to protect them secretly. The four lucky stars went to prison to rescue Dai Geda and went to find his daughter Mina. It happened that Wu Erqi was about to rape her, so they rescued her. Dai Geda was grateful to the four lucky stars and agreed to be a tainted witness to testify against Wu Erqi's crimes, but he tried every means to exonerate Wu Erqi. Police Superintendent Cao also arranged for Banana to be killed by Wu, which inspired the four lucky stars to avenge her. At the end of the film, the Four Lucky Stars and the police arrested Wu Erqi and wiped out the gang he led.