The movie starts off in Prague, where an armed escort is taking a prisoner to police HQ for questioning. Across the road from the HQ, on the belltower of a church, stands Joe (Nicholas Cage), who is busy assembling a sniper rifle. He checks the time - it's almost 9 PM. The prisoner is taken to an interrogation room and made to look at pictures. Joe has a perfect shot of his target through the room window; the only question is time. He waits till precisely 9 PM, when the bell tolls, and takes the fatal shot, killing the prisoner and leaving the cops in disarray.
Returning to his hotel room, through voice-over, he tells us his 4 basic rules - one, never asks questions; two, never take an interest in people outside of work; three, erase every trust; four, know when to get out. His contact - a junkie - enters the room. Joe hands him his money... then tasers him, till he falls unconscious. He injects him with a lethal dose of pure heroin, basically killing him on the spot.
Some time later, he enters Bangkok, Thailand, for his last job. He has to kill four men. At a warehouse, we meet a mob boss, Surat, and his blonde-haired henchman, Aran. They talk about Joe and whether he is reliable. Aran tells Surat that the Russians have recommended him.
Joe arrives at a private house, his safehouse. Needing a "disposable" contact, he goes about the town. He notices a young man, Kong (Shahkrit Yamnarm), trying to sell some American tourists some fake Rolexes, while stealing one of their wallets. He approaches Kong and tells him that he'll pay him 3000 Baht per day (after some haggling) if he helps him out for the week - basically pick stuff up and drop stuff off. He pays him half upfront. Kong agrees readily. Joe tells him that if he's late to the safehouse, he won't get paid for that day.
The next day, Kong shows up on time. Joe gives him a cell phone and a watch for him to keep at all times. His first job is to go to a club and contact a local dancer named Aom and hand her a marked 1000 Baht note and take a briefcase, which he should not open. Fed up with the rules, Kong goes to the club. He sees Aom dancing on the stage and is almost instantly smitten. Seeing the marked note, she beckons him to the back room, where she hands him the briefcase. He rides back to the safehouse on his moped, with Joe tailing him in his car. He jumps a red light and gets chased by a police car, but manages to lose it.
At the safehouse, Joe tells him that he won't get paid for this day as he was chased by the cops. Frustrated, Kong leaves. Joe opens the briefcase and sees a picture of the first mark. Memorizing the face, he burns the picture. He starts to surveil the target.
Kong returns to the club to return the briefcase and sees Aom again. She too seems mildly interested in him. At a Thai kickboxing match, she hands the case to her employer, Surat. Surat looks inside and looks at Joe's list, which includes a syringe of heroin. He orders Aran to give him what he needs.
Kong returns to Joe's safehouse and hands him the case. Seeing a picture of an elephant with its trunk pointing down, he tells Joe that it's bad luck. Joe dismissively shuts the door in his face. Joe then opens the briefcase and takes out the weapons and stuff he requisitioned from Surat, including the heroin syringe.
FIRST HIT: He sits on a motorcycle, preparing an Uzi, waiting for the first guy's car to reach a traffic signal. A little girl, selling garlands, sees him. He turns the traffic light red, causing the target's car to stop at the light. With 30 seconds to spare, he drives to the front of the car and unloads into the target car. When the light turns green, he drives away. Suddenly, the garland girl runs in his way, forcing him to swerve, scratching his arm against some iron beams. He manages to get away.
Needing to dress the wound, he goes to a pharmacy. A deaf-mute young lady, Fon, comes up to him. Seeing his wound, she gives him what he needs. He feels a small attraction for her.
Before going to meet Aom for a case pick-up, Kong buys her expensive earrings, which she loves. On his way to Joe's safehouse, he is attacked by some thugs, who want their cut from him. As they beat him down, the head thug smashes open the briefcase. However, Kong manages to escape with the case. He reaches Joe's safehouse, clutching the case tightly. Joe is upset that the case is open. Kong mentions that the man in the photo (in the case) is a bad man. He asks if Joe's going to kill him. He wants to be like Joe and wants Joe to teach him. Joe attacks him suddenly, holding a knife at his throat. Kong however persists and asks for Joe to teach him the "trade". Joe, seeing himself in Kong, starts the lesson immediately, forcing Kong to defend himself against Joe's knife arm. That was his first lesson.
He shows Kong the ropes, making him aware of his surroundings. He trains him in martial arts and shooting.
SECOND HIT: Kong tells him that the second man, Pramod Juntasa, is a sadistic pimp, who trades in young girls. The hit will take place on the Sheraton Hotel and should appear accidental. Joe sneaks inside the hotel, dressed in black. He sees Juntasa taking a swim. Getting in the pool himself, Joe grabs him and drags him underwater. He drowns him with his bodyguard none the wiser.
Kong's training intensifies. Joe goes to see Fon again at the pharmacy. Asking her co-worker to translate (in sign language), he asks her out for dinner. Coyly, she agrees. After a very spicy dinner, he goes to a local dance show, where she participates. We see a montage of him and Fon, as well as him training Kong.
Kong visits Aom at the club. Surat and Aran watch them. Surat wants to know more about Kong. Aran follows Kong as Kong drives to Joe's safehouse. Kong, noticing the tail, calls Joe. Joe instructs him to put his cell phone inside the bag and to dump it. Kong complies. Aran picks up the bag and takes it back to Surat. Joe calls them on Kong's cell and threatens to kill Surat's wife (who's sitting with them) if they try to mess with him.
Joe goes to meet Fon's mother. He tells her that he's a banker. It starts to rain. Through sign language, Fon tells Joe that her name means, "rain". They hold hands, lovingly. At home, he turns the elephant picture (that Kong told him was bad luck) upside-down, making the trunk point upwards. He even throws away the heroin-filled syringe.
THIRD HIT: Joe looks at the picture. Kong asks him if he's a bad man. Joe replies that he's bad for someone. Joe and Kong get on a boat, in the crowded river, planning to intercept the third target as he approaches them on another boat. However, because of too many witnesses about clicking pictures, he can't take the shot. However, the target notices the silencer sticking out from under the flowers in the boat. Immediately, he instructs his boat driver to flee the scene. Joe and Kong get on another boat and give chase. After crashing into another boat, their boat begins to stall. Seeing a motorcycle rider on the adjacent street, he commandeers it and chases the boat, exchanging fire with the target. His bike takes hits in the fuel tank. He jumps onto the boat, just as his bike explodes. Using the boat engine rudder, he slices off the man's gun hand. Taking the gun from his hand, he executes the target, absolutely stone-faced.
That night, they watch a middle-aged man talk on TV. Kong tells Joe that he is a very good and popular man, who fights crime. Joe doesn't have the heart to tell him that he is the fourth target. He and Fon go to see the Reclining Buddha. As they walk through a park, she hands him a note saying she's very happy with him. She turns coyly away from him. Just then, a couple of gun-toting thugs attack him, but he overpowers them and kills them. Their blood sprays on to Fon. Seeing the blood and the bodies on the ground, she freaks out and recoils away from Joe. She runs away.
He goes to see her at the pharmacy and her house. But she rebuffs him. Mad, he rips off the elephant picture and burns it. He's furious at himself for breaking his own rules. He starts to distance himself from Kong. Seeing the last target is the well-liked and good politician, he asks for more money from Surat, among other things. Surat feels there's too much risk and decides it's time to shut down shop - which means, kill Joe, Kong and Aom.
They grab Aom and force her call Kong. Kong takes a case to Joe, asking if the last target is a bad man too. Joe says he's bad for someone. Opening the case, he sees a sniper rifle inside and a heroin-filled syringe (which he asked for). When Kong goes to the club to see Aom, he's attacked by Surat's thugs. He manages to defend himself, but a gun to Aom's head forces him to surrender. He is then forced to spill where Joe is staying.
FOURTH HIT: A parade for the politician/target is underway. There's massive security around the place, with cops on the ground and snipers in the buildings. Joe sits in a dark, abandoned building with the rifle, preparing himself for the hit. He's heavily conflicted about this job. Through the scope, he sees the many happy faces of the people as they see the politician. At the last moment, he pulls the trigger. The camera tracks the bullet as it reaches the politician's head and we see flashbacks of Joe's previous kills.
Then, we see that it was all in his imagination. He never pulled the trigger. He can't do it. However, the police snipers notice him and open fire at him. He manages to get away, disguising himself as a journalist. At the safehouse, he starts to pack up to get out. Suddenly, he hears a sound. Three of Surat's thugs enter the house, guns drawn. He throws down two bombs and takes refuge in the bathtub. After the explosions, he goes downstairs and interrogates the two surviving (barely) thugs. One doesn't talk, so BANG! The other breaks and tells him Kong's with Surat.
Surat and Aran are in their warehouse, watching the failed assassination. Since they have no answer from their hit squad, Surat feels he may have already killed them. Aran is skeptical.
Joe goes to Fon's house and gives her a last "wai" (bow and touches fingertips together). She runs after him silently as he drives off.
Dragging the surviving hit squad member to the warehouse, he gets past security. Using the guy as a human shield, he kills a lot of Surat's thugs. Inside the warehouse, he knocks off even more gangsters; however, he gets wounded during a firefight with one of the gangsters. Aran and other thugs arm themselves for the fight. The thugs are dispatched immediately. After a short fight, Joe makes Aran hug a grenade to an iron door. Aran gets ripped apart as Joe escapes just barely. Getting to Kong and Aom, he tells them to run. He still has to kill Surat. As Surat and his bodyguards try to escape in a car, he kills the bodyguards and driver. Surat stays low, as Joe stands at a distance waiting for a cowering Surat to show himself. Joe gets in the car and sits beside Surat. His wound looks quite bad. The gunfight has drawn cops to the area. Putting the car in reverse, he lets it move slowly through the gate towards the cops. Pressing Surat's head to his own, he holds his gun to his other temple. After a long and suspenseful wait, he pulls the trigger...
Kong watches from behind the police barricade, distraught.
The movie ends with Kong standing alone at a pier.