In Pittsburgh, a man drives a van into a parking garage across the Allegheny River from PNC Park, dropping a quarter into the meter. Using a rifle, he shoots and kills five people on the river's North Shore Trail from long range before driving off. The Pittsburgh Police, headed by Detective Calvin Emerson, find a shell casing and the quarter used to pay for parking. A fingerprint on the coin belongs to James Barr, an ex-United States Army sniper. When the police raid his house, they find the van, the rifle, and Barr unconscious. During an interrogation by Emerson and District Attorney Alex Rodin, Barr is offered a choice between life in prison in exchange for a full confession or a guaranteed death sentence, as Rodin has never lost a case.

Barr instead writes "Get Jack Reacher" on a notepad. Jack Reacher is a drifter and ex-US Army Military Police Corps investigator and major. He arrives in Pittsburgh after seeing a news broadcast about Barr. Emerson and Rodin deny Jack's request to view the evidence, but agree to let him see Barr, who was attacked by fellow inmates and is now in a coma. Jack meets Barr's defense attorney, Helen Rodin, Alex Rodin's daughter. Helen tells Jack he can see the evidence if he will be her investigator, but Jack retorts that he is not interested in clearing Barr. He reveals that Barr had gone on a killing spree during his tour of duty in Iraq, but was not prosecuted because, unbeknownst to Barr, his military contractor victims had just emerged from a weekend of assaulting numerous women, and the US Army just wanted the whole mess forgotten.

Jack vowed that if Barr tried anything like that again, he would take him down. Jack agrees to investigate if Helen visits the victims' families to learn about the people murdered that day. Jack goes to the crime scene and finds inconsistencies; a trained sniper would have shot from a van on the nearby Fort Duquesne Bridge. An apparently random bar fight makes Jack realize that someone is attempting to strong-arm him into dropping his investigation. After Helen reports her findings about the victims to Jack, he suggests that the owner of local construction company Oline Archer was the real target, while the other victims served as a cover-up. Jack is later framed for the murder of the young woman who was paid to instigate the brawl.

Jack eventually follows up a hunch to a shooting range in neighboring Ohio owned by ex-United States Marine Corps Gunnery Sergeant Martin Cash. The real perpetrators are working for a ruthless construction firm led by an ex-Soviet prisoner known only as Zek Chelovek ("prisoner human being"). The gang kidnaps Helen with the aid of Detective Emerson and holds her hostage at a quarry. Jack kills the henchmen and Emerson with Cash's help, before confronting the Zek. The Zek points out that Jack has killed all who could have testified against him and doubts that he will be convicted, while also stating that an American prison would be easy compared to his previous incarceration in Siberia.

Convinced, Jack shoots him in the head. When Barr awakens from his coma, he tells Helen that he has no memory of the crime, but believes that he must be guilty. Barr's mental reconstruction of how he would have committed the shootings matches Jack's theory. A remorseful Barr is willing to accept both responsibility and punishment. However, Helen is confident he will be cleared. Meanwhile, Jack, sitting on a bus, overhears a man verbally and physically abusing a young woman and gets up to confront him.

In a small Indiana city, a lone gunman in a parking garage calmly fires into a rush-hour crowd in a public plaza, committing a massacre of five apparently random victims with six shots. The shooter leaves a perfect trail behind for the police to quickly track him down. Evidence from the scene, of a shell case and a quarter bearing the same fingerprints, points clearly to James Barr, a former Army infantry sniper. He is arrested, but will only say two things to the police: "They got the wrong guy," and "Get Jack Reacher for me." Reacher, a former Army military police officer and now a drifter, is 1,500 miles (2,400 km) away, but sees the news on CNN and gets on a bus to Indiana. Reacher has no job, no home, no car, and a shrinking savings account from his past military pay. Although Reacher has a nomadic existence, what he does have is sharp moral clarity.

Instead of clearing Barr, Reacher wants to assist the prosecution in convicting him. Reacher is the last person Barr would want to see for good reason. When Reacher was an investigating military policeman years past, Barr had gone on a killing spree similar to the Indiana shootout, murdering four men during the Gulf War in Kuwait City. Convoluted military politics and a technicality let Barr walk free. Reacher swore he would track the sniper down if he ever tried it again. Reacher believes Barr is guilty, but Barr's sister Rosemary is convinced of her brother's innocence and entreats lawyer Helen Rodin to defend her brother. Helen's father is the district attorney who will prosecute the case. When Reacher arrives in Indiana, Barr has been beaten so badly while in prison that he cannot remember anything about the day of the murders, leaving Reacher to form his own conclusions with the available evidence. The local NBC news reporter, Ann Yanni, is also looking for more information, and Reacher is more than willing to include her in his investigation, in exchange for the use of her car and a guaranteed public exposé on the Barr case. Reacher knows that 35 yards, the parking garage shooting distance to the victims, is point-blank range for a trained military sniper like Barr. Reacher also knows the shooter missed one shot on purpose, giving Reacher one shot at the truth.

Reacher drives to Kentucky to the shooting range where the sniper practiced and learns some interesting facts from Gunny Samuel Cash, the former US Marine who owns the shooting range, which make him doubt the solidity of the presumably airtight case against Barr. Cash is unwilling to reveal information or his records to Reacher, but grudgingly agrees to talk if Reacher is able to hit a paper target dead center at 300 yards with one shot. After he succeeds, Reacher is shown 32 sheets of target paper from three years' worth of Barr's practice shootings at his range, every single sheet with dead-on maximum scores. Reacher, given his expertise with firearms and experience as a military investigator, quickly deduces that the target sheets were faked by using a handgun at point blank range.

After the visit to the shooting range, Reacher adds Cash's information to the case evidence. Helen and Rosemary sift through the clues in a riveting analysis and finally get Reacher to conclude that Barr is innocent, which means someone set up Barr as the sniper. Someone is also trying to get Reacher off the case, which formerly seemed a slam-dunk, but is now falling apart. Reacher is teamed with Helen, working against a prosecution team that has an explosive secret of its own. Reacher gets closer to the unseen enemy pulling the strings, leading him to the real perpetrators, a Russian gang masquerading as legitimate businessmen. The gang's 80-year-old capo spent much of his life in one of the infamous Soviet gulags and is known only as the Zec (prisoner). Reacher outwits the mob guards in the Russian gang's fortress, efficiently and brutally dispatching five hoods before confronting the boss and forcing him to come clean on the conspiracy from beginning to end: covering up the murder of the one victim he wants dead with the other four victims. The real killer is shot, the Zec chooses to confess to the authorities who arrive on the scene, Barr goes free, and Reacher moves on.

The novel was adapted into the 2012 film Jack Reacher. Written and directed by Christopher McQuarrie, the film stars Tom Cruise as the title character. The film entered production in October 2011, and concluded in January 2012. The setting, however, was changed from Indiana to Pittsburgh, Pennsylvania, where the film was shot in its entirety. Lee Child has a cameo role as the property desk officer in the police station after Reacher's arrest. The film was released on 21 December 2012. A premiere at Pittsburgh's Southside Works megaplex on 15 December, to have been attended by the film's stars and Child, was postponed following the Sandy Hook Elementary School shooting.^{[citation needed]}