A man, Smith (Clive Owen) sits on a bench ready to eat a carrot. A woman in a yellow coat runs past chased by a man driving an old black Camaro. The carrot eater follows and stops the man from killing the woman inside an abandoned factory building by driving the carrot through the man's head. Suddenly several other gun shooting attackers arrive and Smith fires back killing several. The hero and the woman get to another room where she gives birth. More men continue to attack, led by a balding, bespectacled man, Hertz (Paul Giamatti) . As Smith, the woman and her baby try to escape the woman is killed and Smith flees with the baby.
Stopping in a washroom, Smith has a fight with a well dressed hitman, leaves him alive. On a bus he uses his sock to make a cap for the newborn baby. Smith then leaves the baby in a park on a rotating ride. As a bystander arrives to pick up the infant she is shot by Hertz from long range using a sniper rifle. Smith manages to rescue the baby and takes off again.
He goes to a church which is actually a brothel. There he finds DQ (Monica Bellucci), a lactating wet nurse hooker and offers her $5000 to look after the baby. She refuses. Hertz fondles the dead mother's breast and realizes the baby will need milk, he to decides to find a lactating hooker and soon arrives at DQ's. He tortures her for info on Smith, who then returns. Smith shoots Hertz and some henchmen, and escapes stealing a BMW. He then gets upset at a Mercedes driver who doesn't signal lane changes and rams the other car.
Hertz is still alive and calls someone for 50 more men.
In a downtown area, Smith tries to buy bullets with some change and a blood donor cheque. DQ earns some cash by giving a man a BJ in an alley. Smith takes DQ and the baby to his place - a loft in a nondescript industrial building. The door lock is activated by a rat.
Driving with two assistants, Hertz arrives at the same building. Apparently Hertz was an FBI Forensic Behaviour Consultant and knows what people will do.
In the loft apt, Smith fashions a diaper from a newspaper and decides to call the baby Oliver as in Twist. Smith and DQ notice the baby calms down when watching heavy metal rock on TV and cries when a politician speaks on another channel. Suddenly dozens of attackers arrive. DQ escapes down a dumbwaiter as Smith and the baby escape in a wildly stylized hail of bullets. Hertz and his henchmen give chase with dogs but Smith eludes them by using the baby's diaper as a decoy.
Smith decides the baby got used to heavy metal music while in the womb and they go to the local metal club. They go upstairs and find dead men in the office as well as a photo of three women in yellow dresses, one was the baby's mother. Next door Smith and DQ overhear a suit talking on the phone about the dead women. They then find a lab room with a fridge full of samples from one donor. Smith suspects the babies are being used for someone's bone marrow transplants.
Smith, DQ and the baby return to a hotel room, Smith decides to wait and watch TV, having called all the major news outlets with the scoop. DQ is a saddened by the memory of her own child. Smith and DQ kiss and have sex. They are interrupted by several swat-team type gunmen but Smith shoots them all as he stays coupled with DQ.
Smith brings DQ and the infant to an armoury, planning to put the two into an M-24 tank for safety. He gives her a weapon then causes a distraction by spanking a woman scolding her child to allow DQ to get inside. Smith loads up to go to the Hammerson gun works.
Hertz and Hammerson (Stephen McHattie) discuss a British gun wiz as Smith lurks closeby in the gun factory warehouse. Once again a wild gunfight erupts and Smith and Hertz trade verbal barbs. Hertz reveals he knows Smith's wife was killed in a fast food place. Smith goes into action using several automatic weapons he has rigged to fire with strings.
Back at the tank Smith explains to DQ that Hammerson is the rich guy involved. In a newspaper he notices the front page story of Senator Rutledge (Daniel Pilon) and realizes Rutledge is the one who needs the baby and his firearms bill will put Hammerson out of business.
On the street the three are attacked by a black SUV. Smith takes the baby and jumps into a red BMW and a wild car chase starts. Smith takes out the SUV but the baby falls out onto the road. A black van full of gunmen arrive, Smith shoots them all but Hertz arrives in his limo and drives over the baby. But it is only a doll and tape recorder.
Smith calls the Senator's aide (from the earlier washroom fight) asking for a meeting. The two men meet in a airport washroom and they go up into the Senator's campaign charter jet. Smith confronts the ailing politician. As they talk Smith notices white dog hair lint on the other man's pants. He grabs Rutledge realizing he had made a deal with Hammerson. Hammerson and Hertz come from the rear of the plane. Smith takes Rutledge hostage in the plane's cargo hold. He tells the Senator that killing Rutledge will ensure Congress passes the anti-gun bill, then shoots him in the head. Smith bails out with a parachute chased by attackers, they have a gunfight in mid air. Smith manages to push the Senator's aide into the blades of a helicopter.
On the ground a wounded Smith observes the body parts and dead bodies. He collapses in an industrial building. As he wakens Hertz is standing over him with a gun. DQ walks warily in front of a church.
A beaten Smith is dragged into a basement den. As Hammerson watches, Hertz breaks Smith's fingers, torturing him in order to learn where DQ and the baby are. Hertz explains that the Senator's plane crashed, so their coverup is underway. Meanwhile, DQ boards an old green bus to "Wherever". Hertz increases the violence and brings a scalpel towards Smith's eye. Smith suddenly manages to break free and kills the two henchmen and Hammerson. Smith staggers upstairs and collapses beside a fireplace. Hertz comes near to gloat, then notices Smith's hand in front of the fire, with bullets in between each finger pointed toward him. The heat causes the bullets to fire, and Hertz is shot in the chest. Not quite dead, Hertz rises, but finally Smith finishes him off.
With heavily bandaged hands, Smith waits for the green bus in front of the church. He rides the bus full of hippies and gets off at an ice cream shop. Oliver, the baby, is there, safe; DQ is the waitress, and Smith kisses her. Suddenly, some scruffy, armed robbers break in; Smith is able to kill them all, using a carrot to help fire a gun with his bandaged hands.