Diagnosed with a terminal illness, Cheung (Andy Lau) is given four weeks to live. As he eats at a diner, Inspector Ho Sheung-Sang (Lau Ching-Wan), a police negotiator, has been called to the scene of a bank robbery standoff that's being bungled by his inept boss, Wong Kai-Fat. Listening to a negotiation between Wong and the robbers, Ho feels that the robbers are professionals and goes in the negotiate with the robbers, but finds them nervous and arguing with each other. After goading the robbers, a man amongst the hostages stands up and shoots the robbers dead. Claiming to be an off duty officer, Inspector Ho recognizes his sniffling as the robber talking to Wong before he went in. As the police rush in, Inspector Ho leaves the crime scene to grab some breakfast. In the crowd, an old man takes pictures of him.

Cheung has taken an interest in Ho, who he discovers was a former member SDU. He then stages his own robbery to get in contact with him. He scares away the personnel of a finance company and takes the manager hostage, stuffing his jacket full of diamonds and money. Cheung is met with another negotiator and Wong, so he drags out the negotiation longer until Ho is called to the scene. When Ho finally appears, Cheung proposes to play a three-day long game. Cheung shoots the manager and Ho and the police give chase until Cheung stalls them enough to make an escape with a bomb, which turns out to be fake. The manager also reappears unharmed, having been shot with a paintball.

In the confusion of trying to track down Cheung down, Cheung makes his escape by posing as SDU and a cop. Ho is the lone person giving chase, but meets with Cheung after posing as a taxi driver. Amazed at Ho's ploy, he'll admit defeat at the game if Ho can take him to the police station before three days are over. Cheung then pulls a gun a starts shooting out the window to test Ho's reaction. When Ho slams the brakes, Cheung flees and jumps aboard to mini-bus. When stopped at a police road block, Cheung avoids them by sitting next to a woman on the bus and pretending to be her boyfriend. Having seen his gun, she cooperates, letting him put his sunglasses on her and taking one of the earphones on her music player. Leaving the road block, he gets off. The woman gets off as well, giving Cheung his sunglasses back. Dealing with the aftermath of letting Cheung get away, Ho realizes that they haven't looked into the finance company that Cheung held up.

Meanwhile, Baldy is having a diamond that he had stolen appraised by the old man who was taking pictures of Ho earlier. The two men who stole the diamond for him ask for more money. Baldy pulls out his gun and points it at the old man, who confirms it's authenticity. Baldy then kills the two men and takes the diamond. Ho returns to the finance company for further questioning, and sees Baldy entering the office space next door. He tries to get in, but it stopped by one of Baldy's men.

As Cheung watches a video feed of the vault at Baldy's office, he puts a screw inside a box. As he takes a drink, he coughs out blood, meaning his condition is worsening with internal bleeding. Ho tries to reason why Cheung held up the finance company and not the jewelry or antique stores in the building and why Cheung chose him for his game. As he leaves for the day, Ho receives a Cheung's package. He meets up with his friend, the Head of Interpol, who's given him information on Baldy, who's wanted for possession of explosives, and one Peter Cheung, the old man who took pictures of him and appraised the diamond. Peter Cheung was Baldy's boss until Baldy kicked him out.

More focused on the case, he skips having dinner with her to break into Baldy's office. When trying to get in, he's turned away by security, who's told by Wong that he's not on the case. Ho sneaks in anyway as Cheung watches him on video feeds. Using a packet of dairy creamer he took from security, Ho enters the finance company and finds the grate that Cheung's screw comes from. Going through the vents, he finds men that Baldy had to keep an eye on the diamond and Cheung's video equipment, which Cheung uses to give away Ho's entrance. Ho tries to escape, but Cheung calls him and leads him to an emergency door that turns out to be a dead end. Ho tries to pick the lock on the emergency door, which is opened by the guy who he encountered earlier when he tried to get into the office.

Knowing that Baldy will come down to get the diamond out of there, Cheung reveals a car identical to Baldy's. Cheung takes the diamond, and Ho and Baldy's men give chase. As both Ho and Cheung get fired on by Baldy's men, the two are amazed at each other's resilience. They work together to get out of the predicament, but Ho realizes that now he's the one with a gun and Cheung is the one driving. Cheung reiterates that if Ho can get him to the police station, he wins. However, Cheung drives the car into a wall. After the collision, both men try to get the diamond. Cheung walks away with it, but not before dropping his painkilling pills.

Wong, who thinks the pills are Ho's, tries to talk to him. Ho's Interpol friend comes and brings him more information on Peter Cheung, who died a year ago, and a photo of Cheung, his son. Cheung, who has been masquerading as his father, sends Baldy proof that he has the diamond and will give it back for $20 million HK. Ho meets with Cheung at the diner, asking why he's still playing with him even though he's got the diamond and has avenged his father. Ho gives him a minute to give him a good reason not to arrest him now. Cheung wants to have Ho arrest Baldy, but realizes that Ho isn't doing this for fame or a promotion. Cheung then taunts Ho, saying that he's beating Ho in their little game. Ho lets him leave so they can finish their game.

Cheung takes a bus, where he meets the woman from earlier again. The bus is pulled over by the police, and the woman invites Cheung over like before. She takes him to a restaurant, but he leaves when he starts coughing up blood.

Cheung has set the place to make the exchange for the diamond at a bowling alley, which is being staked out by Ho, Wong and the police. Ho bumps into the same henchman he's been running into, who still doesn't recognize him. Cheung has Ho make the exchange for him, but Baldy has Cheung, who's disguised as a woman, get the money. Ho tries to get Wong to check in on the money exchange, but Wong doesn't get the message. Cheung, who's taken out Baldy's man and the cop while getting the money, takes off his disguise and makes the exchange himself, telling Ho that the "woman" is conning him. Baldy takes Cheung's bag, which holds a bowling ball, and throws it on the floor in rage, revealing diamonds inside. The police move in and arrest Baldy for stealing the diamonds from the finance company next door.

Cheung leaves the scene with the diamond and the money. Ho catches up with Cheung, finally arrests him and prepares to take him to the police station. Cheung reveals another "bomb" and pulls out another detonator like the one before. Ho calls him on his bluff and presses the detonator, which starts a timer on the bomb. Cheung, who know's he's going to die soon, wishes that he not die in a jail cell, so Ho stops and gets out of the car. The timer reaches zero, which starts up the car and Cheung escapes again. When Wong berates Ho for letting Cheung get away again, Ho simply ignores him and goes to grab something to eat. Cheung drives away, smiling.

Ho reads in the newspaper that someone using his name donated $20 million HK to a children's cancer foundation. He hops on a bus and meets the woman Cheung kept running into wearing the diamond. Ho admires the diamond, though she brushes it off as something cheap. He asks who gave it to her, and she says she hasn't seen him in a while. Ho tells her to hold on to it.