In Mexico City criminal gangs are kidnapping rich people and their families for the lucrative ransom money. On the advice of his lawyer, Mexican businessman Samuel Ramos (Marc Anthony), like other wealthy people, hires a bodyguard to protect his nine-year-old daughter Lupita "Pita" Ramos (Dakota Fanning), and to be able to renew the family's ransom insurance. The head of an executive security firm, Paul Rayburn (Christopher Walken) suggests that Ramos hire Rayburn's old CIA buddy John Creasy (Denzel Washington).
Ramos is impressed with Creasy's résumé, which includes a variety of counterinsurgency and counter-terrorism activities. However, Creasy has since become uninterested in life. He is burned out from all the death and horror and has taken to alcoholism to ease the burden. Creasy is not interested in bodyguard work and even less with the youngster, but he needs a job. Creasy's initial impatience slowly fades as he finds himself opening up to the child. He replaces her parents in their absence, giving her advice and coaching her in her swimming lessons and competitions. He helps her overcome her fear of starting guns, turning it into an eagerness to move at the sound of one. Pita gives Creasy an emblem of St. Jude, the patron saint of lost causes, saying she bought it for him with her own money.
Shortly after delivering her to a piano audition, Creasy notices a suspicious vehicle driving past and a pair of police cars blocking off the surrounding street. As Pita emerges from the building and freezes, Creasy pulls out his gun and fires a shot into the air, similar to a starting gun, startling her to running away. During the ensuing gun battle, four of the attackers are fatally shot, including two corrupt police officers. The battle leaves Creasy seriously wounded and unconscious, allowing the surviving kidnappers to grab the child.
Shortly after the kidnapping, gang leader Daniel Sanchez demands a dead drop ransom of several million dollars in exchange for Pita. The drop goes badly, the kidnappers are ambushed by a corrupt police gang called "La Hermandad", and the money is stolen. Sanchez notifies Pita's parents that the child will not be released, and everyone assumes she is dead. It is during this time that Creasy is made into public spectacle by the police chief while unconscious in his hospital bed, claiming he has murdered two judicial police officers.
Rayburn shows up at the hospital after hearing the news and aids in helping Creasy escape to a safe house to further recover from his injuries. After Creasy recovers from his wounds, he returns to Pita's home and promises her mother that he will kill everyone who was involved in, or profited from, the kidnapping. Creasy kidnaps and tortures several "La Hermandad" members, forcing them to reveal the names of the people involved in the kidnapping ring. Also Ramos admits setting the whole kidnapping up in order to get insurance money as he was bankrupt. He then commits suicide with a gun provided by Creasy.
With Rayburn's help, Creasy buys a variety of guns, explosives, and a rocket-propelled grenade launcher. He uses the weaponry to kill a number of corrupt Mexican police officers and high-ranking law enforcement officials who were involved in the kidnapping. After Creasy learns where Sanchez lives, he fights his way into Sanchez's apartment. Although Creasy is wounded in the gunfire, he gains the upper hand, and captures and tortures Sanchez's brother and ex-wife.
To save his brother, Sanchez admits that Pita is alive, and offers to exchange her for his brother and Creasy. Creasy agrees, and travels with Pita's mother to a remote rural area to make the exchange. After Pita runs to her mother, Creasy is hustled into a car, which speeds off. As Creasy succumbs to his gunshot wounds, he drops the emblem of St. Jude that Pita gave him. An epilogue reveals that Sanchez was killed during a police raid that same day, his location discovered by special operations police who were electronically following Creasy's movements and actions.
