In 1991, Ming, a Triad member, followed the instructions of Chan, a Triad boss, and joined the police force as an undercover agent. Over seven years, he had been promoted to the rank of sergeant. In 1992, police cadet Yan was forced to drop out of the academy and became an undercover agent for the Triads, where his current boss is also Chan. In 2002, the Serious Crime Unit obtained intelligence about a drug transaction and identified Chan as the target. However, intelligence was leaked from both sides, resulting in the failure of the police operation and the unsuccessful drug deal, leaving both parties defeated and exposing the fact that both sides had undercover agents. This triggered the determination of both sides to eliminate their moles. Yan and Ming have fates that are both different and similar, and they face severe tests on their journey through the "Infernal Affairs."