Prohibition in the United States led to an organized crime wave in the 1920s and early 1930s. Various gangs bootleg vast amounts of alcohol and enforce their business with violence and extortion. The problem is most serious in Chicago, where gang leader Al Capone (Robert De Niro) supplies low-quality liquor at high prices. Treasury Department agent Eliot Ness (Kevin Costner) is put in charge of leading the crusade against Capone and his empire. Early warehouse raids fail due to corrupt officers in the Police Department.
Ness solicits help from Jim Malone (Sean Connery), an incorruptible Irish-American police officer. Malone advises Ness to find more members to their new team from the police academy to ensure that no one betrays them to Capone. An Italian-American trainee George Stone, formerly Giuseppe Petri (Andy Garcia) is then enlisted due to his superior marksmanship. Along with an accountant Oscar Wallace (Charles Martin Smith) assigned to Ness from Washington, Ness has formed a new team to deal with Capone.