In 2010, Cheuk Hoi secretly travels to Bangkok to conduct a drug operation with his undercover agents in Thailand. As they finish their mission and are about to leave Thailand, an explosion causes all of Michael's undercover agents to die in a sea of fire. At the same time, Michael's good friend, CIB Inspector Eddie Cheung, is killed for unknown reasons, leaving Michael in charge of a female cop who is not yet officially an undercover. Michael knows that Eddie still has undercover agents whose identities have been deleted. To learn the truth about Eddie's death, Michael must find all of the undercovers. He and Priscilla begin to search for the undercover agents.

一次行動，CIB高級督察卓凱的五位臥底及一位好友先後喪命，遂與新紮臥底鄭淑梅展開拯救臥底的行動。另一邊廂，長興新任龍頭魏德信整頓組織，頭目覃歡喜另起爐灶，命名「福和」，與德信對抗。卓凱派淑梅混入長興，與德信的手下施嘉莉、徐天堂及歡喜的得力助手樂少鋒交手。卓凱發覺德信背後有黑警支持，決心把德信的惡勢力連根拔起。德信拼死反撲，將一眾臥底列入「必殺」名單！