In 1930, during Prohibition, the notorious gangland kingpin Al Capone supplies illegal liquor and has nearly the entire city of Chicago under his control. Bureau of Prohibition agent Eliot Ness has been tasked with bringing a stop to Capone's activities, but his first attempt at a liquor raid fails due to corrupt policemen tipping Capone off. He then has a chance meeting with a veteran Irish-American officer, Jimmy Malone, who opposes the rampant corruption and offers to help Ness, suggesting they find a man from the police academy who has not yet come under Capone's influence and still believes in the idealistic aspects of law enforcement. They recruit Italian-American trainee George Stone (birth name Giuseppe Petri) for his superior marksmanship and integrity. Joined by accountant Oscar Wallace, assigned to Ness from Washington, D.C., they conduct a successful raid on a Capone liquor cache and start to gain positive publicity, with the press dubbing them "The Untouchables." Capone later kills the person in charge of the cache as a warning to his other subordinates.
Wallace discovers that Capone has not filed an income tax return for four years and suggests that the team try to build a tax evasion case against him (as Capone's network keeps him well-insulated from his other crimes). An alderman offers Ness a bribe to drop his investigation, but Ness angrily refuses it. After Capone's enforcer Frank Nitti threatens Ness's family, Ness immediately moves his wife and daughter to a safe house. In a subsequent raid on the Canadian border, Ness and his team intercept an incoming liquor shipment. They kill several gangsters and capture a Capone bookkeeper named George, whom they eventually persuade to collaborate. Back in Chicago, as Wallace escorts George from the police station to a safe house, a disguised Nitti shoots both of them dead. Ness confronts Capone at the Lexington Hotel after the murders, but Malone intervenes, urging Ness to focus on persuading the district attorney not to dismiss the charges against Capone.
Malone realizes that police chief Mike Dorsett sold out Wallace and George, and, in a fight with Dorsett, forces him to reveal the whereabouts of Capone's head bookkeeper, Walter Payne. That evening, one of Capone's men breaks into Malone's apartment; Malone chases him out with a shotgun, but falls victim to a Tommy gun ambush by Nitti. Shortly afterwards, Ness and Stone arrive at the apartment to find Malone mortally wounded; before he dies, Malone shows them which train Payne will take out of town. As Ness and Stone await Payne's arrival at Union Station, they see a young mother with two suitcases and a child in a carriage laboriously climbing the lobby steps. Ness ultimately decides to assist her, but the gangsters who are guarding Payne appear as Ness and the woman reach the top of the stairs, and a bloody shootout takes place. Though outnumbered, Ness and Stone manage to capture Payne alive and kill all his escorts without harm to the mother or the child.
Later, when Payne testifies at Capone's trial, Ness observes that Capone appears strangely calm and that Nitti is wearing a gun in the courtroom. The bailiff removes Nitti and searches him, finding a note from Chicago Mayor William Hale Thompson which effectively permits him to carry the weapon. However, Ness sees Malone's address written on a matchbook in Nitti's possession and realizes that Nitti is Malone's killer. Panicked, Nitti shoots the bailiff before fleeing to the courthouse roof as Ness pursues him. After Nitti expresses his contempt for Malone and gloats that he will never be convicted for the murder, Ness pushes Nitti off the roof to his death.
Stone gives Ness a list, taken from Nitti's coat, which shows that the jurors in the trial have been bribed. Behind closed doors, Ness persuades the judge to switch Capone's jury with one hearing an unrelated divorce case. This prompts Capone's lawyer to enter a guilty plea, although Capone is outraged and violently objects. Capone is later found guilty of tax evasion and sentenced to eleven years in prison. On the day Capone begins serving his sentence, Ness closes up his office, giving Malone's St. Jude medallion and callbox key to Stone as a farewell present. As Ness leaves the police station, a reporter mentions the probable repeal of Prohibition, asking Ness what he will do in that case. Ness replies, "I think I'll have a drink."
Kevin Costner as Eliot Ness
Charles Martin Smith as Oscar Wallace
Andy García as George Stone (formerly Giuseppe Petri)
Robert De Niro as Al Capone
Sean Connery as Jimmy Malone
Patricia Clarkson as Catherine Ness
Billy Drago as Frank Nitti
Richard Bradford as Chief Mike Dorsett
Jack Kehoe as Walter Payne
Brad Sullivan as George
Del Close as Alderman
Vito D'Ambrosio as Bowtie Driver
Clifton James as District Attorney George E. Q. Johnson (uncredited)
Don Patrick Harvey as Officer Preseuski
based on the book of the same name (1957)