At a busy traffic junction in front of a jewellery shop, an armoured truck carrying $100 million is ambushed by the ruthless Ronin Gang, who unleash explosives as the convoy stops. The strike leaves all security guards dead except one, and takes a heavy toll on responding police officers. The blasts also claim several innocent lives, including a shopper inside the shop. Six months later, the same gang resurfaces in Hong Kong, wounding a group of police officers—two of them critically—and rekindling the demand for justice. The tragedy is clear: the dead woman in the jewellery shop was CID detective Chan Chun's fiancée, and the injured officers are led by Trainee Inspector Carson Fong Yik-wai. Both Chan and Fong vow to repay the victims.
From there, a determined chase unfolds. The police capture one of the gang’s men, [Little Tiger], who is forced to guide them to the gang’s hideout. A dramatic pursuit follows, culminating in the gang killing Tiger and wounding Chan plus several of his colleagues. As the investigation deepens, a rookie cop named Wai King-ho finds his loyalties tested: his older brother, Wai King Tat, has been missing for an extended period and is believed by many to have slid into the gang while undercover. Suspicion falls on Wai, and he is suspended from duty. Chan and Fong press him for answers, but an unlikely alliance forms after Wai is rescued in a nightclub confrontation.
The plot thickens when the surviving security guard is revealed to be Ho Wing-keung, the manager of the security company who colluded with the gang to pull off the armored-van robbery. He is tracked to a psychiatric hospital, where he feigns a mental breakdown to throw investigators off his trail. Chan, Fong, and Wai reach the hospital too late to stop his escape, as the gang threatens Ho’s family to force him to cooperate. On their pursuit, the trio become entangled in an accident scene where the gang hijacks a mini-bus filled with children and takes Wai hostage, forcing Chan and Fong to chase after the vehicle in a separate car.
The confrontation escalates into a prolonged showdown at multiple fronts. Wai’s undercover brother is revealed, but he is killed by the gang member [Tien Yeng Seng]. Ho is killed in the crossfire by [Tien Yeng Seng], leaving Wai handcuffed to the bus with the children and a timed bomb. Chan and Fong, meanwhile, are forced to retrieve the stolen cash from the gang’s mastermind at a Tamar funfair while bound to their own timed device. Wai ultimately frees himself and rescues the children, only to discover the bomb is a ruse, suggesting the Ronin had no intention of harming the kids.
As the tension builds, Chan and Fong approach a man with a luggage bag at the fair. The man turns out to be Chief Inspector Sam Mok, who is leading the operation at the fair. They seize him and escape their own peril, but the dramatic escape leads to a fatal twist when Sam attempts to shoot them, prompting Chan and Fong to throw him over a railing. The fingerprints of murder stick to Chan and Fong, who become suspects themselves.
With crucial help from Leung Hoi Lam of the Police Intelligence, the truth begins to crystallize: the real mastermind is Senior Superintendent Cheung Man Yiu. He engineered the initial armoured-truck heist, turned the gang against itself, killed several members, and stashed the money in a car at the Police Headquarters car park. The remaining four gang members arrive at the HQ in a police van, pretending to be officers, in a final bid to reclaim the loot. Meanwhile, Senior Superintendent Mark Law Pui Keung starts to dig into the tangled web of deception.
The climactic sequence unfolds in a drawn-out battle at the Police Headquarters. Dozens of lives are lost—including several criminals and several officers—and Wai is killed by [Tien Yeng Seng]. Chan and Fong resist the urge to take revenge and stay true to their oath to arrest, not kill. Cheung responds by hurling a bag of grenades, killing Tien and destroying the remaining money, while Chan and Fong make their escape with Wai’s body. In a final sting, Cheung is lured into an ambulance, where he is shot dead by Chan and Fong as he attempts to ambush [Mark Law Pui Keung] again. The two officers walk away from the chaos to resume their police duties, stepped back into the roles of detective and beat cop, and they also help care for Wai’s grandmother, Ho's Grandma in the aftermath of the violence.
