Chan Wing-Yan, portrayed by Tony Leung Chiu Wai, is a dedicated police officer who infiltrates an organized crime triad. The only person in the know about his covert mission is his direct superior, Superintendent Wong, played by Anthony Wong Chau-Sang. Meanwhile, Lau Kin-Ming, played by Andy Lau, is secretly working within the Hong Kong Police Force on behalf of the formidable gang leader, Hon Sam, portrayed by Eric Tsang. Each man is a spy for their respective organizations, aiming to gain the upper hand in a relentless battle for power.
As ten years pass, Chan endures immense pressure and psychological strain due to his undercover assignment, while Lau ascends the ranks within the police department with remarkable speed. He cleverly communicates with Wong using Morse code on a hidden radio frequency embedded in a plaster cast on his hand, alerting him to an impending drug deal involving Sam and the Thais. The exchange is set to occur in an apartment, although the drugs themselves will be handed over a few miles off the coast.
However, Lau's loyalty to Sam runs deep; he sends an SMS to all local cell phones using his private data card, which ultimately gives Sam the advance warning needed to destroy the evidence. In the wake of this chaotic incident, both Wong and Sam set out to uncover the moles within their own ranks. With a promotion, Lau is reassigned to the internal affairs department, where he intensifies his quest to flush out the rogue officer in the police division.
Chan, unaware of Lau's true identity, trails Sam to gather intelligence about the men present during the drug deal. A tense exchange of information occurs in a dimly lit theater, but Chan is momentarily distracted by a phone call, allowing Lau to narrowly escape.
Worried for Chan's safety, Wong plans to extract him from his undercover role, yet remains oblivious to Lau's vigilant monitoring of Chan. Wong meets Chan at their usual rendezvous, but Lau quickly informs Sam, leading to Wong's capture and brutal demise as he is thrown from a building after refusing to disclose Chan's status.
As the story unfolds, Lau seizes Wong's phone and reaches out to Chan with a proposition to thwart Sam's drug deal. Despite the plan's success resulting in numerous arrests, Lau ultimately betrays Sam and kills him. While it appears that order has been restored, Chan uncovers that Lau is the actual mole, leading to his immediate departure.
In a gripping confrontation on the rooftop where Wong met his end, Chan disarms Lau and raises a gun to his head, challenging his rival's plea for redemption. Just as the tension peaks, Inspector B, played by Gordon Lam, arrives and demands Chan lower his weapon. In a shocking twist, as Chan backs away into the lift, he is unexpectedly shot by B, who reveals himself as another mole aligned with Hon. As they descend, Lau murders B, eager to erase his past and fit into his envisioned role as an upstanding cop.
When they reach the ground floor, Lau reveals his identity to the police, effectively sealing his fate. Months after the tragic death of Chan, his psychiatrist Lee (Kelly Chen), who previously helped him maintain his sanity, discovers the truth about his undercover status. The narrative takes an unexpected turn as B is made the scapegoat for Lau's deception, leading to the closing of the investigation. At Chan's funeral, Lau pays his respects, reflecting on the choices that shaped his fate.
In a final note on the story's conclusion, an alternative ending was produced for audiences in Mainland China and Malaysia, where Lau's identity is uncovered, resulting in his arrest by the police, further complicating the narrative for any potential sequels.
