Infernal Affairs focuses on a police officer named Chen Wing-Yan (Tony Leung Chiu-Wai), who goes deep undercover into the Triad society, and Triad member Lau Kin-Ming (Andy Lau Tak-Wah), who infiltrates the police department. Each mole was planted by the rival organization to gain an advantage in intelligence over the other side. The more the moles become involved in their undercover lives, the more issues they have to cope with. The film follows their struggles with their double identities and their race to uncover the other mole. Eventually, they reach a deal to foil a drug deal orchestrated by an antagonist gangster named Hon Sam (Eric Tsang Chi-Wai), resulting in his death. However, Yan discovers that Ming was the mole, leading to a confrontation on a rooftop. Yan holds a gun to Ming's head, but is shot and killed by another mole within the police force, Inspector B (Gordon Lam Ka-Tung). Ming then shoots Inspector B dead and turns himself in to the police. The alternate ending shows Ming being arrested after being informed that evidence of his role as a mole has been found. He willingly surrenders and is taken into custody.

Infernal Affairs II is a crime thriller set in Hong Kong during the 1990s. The movie follows the story of an undercover police officer who infiltrates a powerful triad organization. As he rises through the ranks, he struggles to maintain his identity while facing constant threats from both sides. The movie explores themes of loyalty, betrayal, and the blurred lines between good and evil.

Infernal Affairs III is the gripping conclusion to the Hong Kong crime trilogy. The story follows the intense battle between a corrupt cop and a gangster within the criminal underworld. As the protagonist navigates the dangerous world of drug trafficking and violence, he faces the consequences of his choices and struggles to maintain his identity as a policeman. With its captivating plot and intense action sequences, Infernal Affairs III is a must-watch for fans of action-packed crime dramas.