1934. The Depression was over 4 years old, and Al Capone was in Alcatraz. Many of the rackets had seemingly legit fronts, such as Midwest Enterprises, Inc...the president is Luigi Renaldo, former lieutenant for Capone. Renaldo is going to Florida on business, and leaving his 2nd-in-command in charge: his Enforcer Paolo Rienzi. But, unbeknownst to his boss, Rienzi tells Tubby to get a couple of guys and work over their accountant, William Norbert, who wanted to retire from the rackets; (nobody is ever allowed to retire). William is a mild-mannered sort, 42, with a wife and kid; is also a genius with numbers which he keeps in his head, and he never makes a mathematical mistake; (sort of like a computer, before they were invented). They rough up William with some brass knuckles.
Florida was the gambling mecca of America; (this is long before Las Vegas was built). Every month, at the Club Sorrento, the "Big 5" bosses would meet: Luigi Renaldo (Chicago), Charles "Fingers" Maccioli (Seattle), Marcus Campanello (K.C.), Al Pacciano (NYC) and "Little" Pete Stefano (Cincinnati). $1.5-million in profits is divided among them, with nary any paperwork; (they'd learned their lesson since Big Al got convicted on income tax evasion...they will leave no paper trail for the Feds to follow).
Meanwhile, back in Chicago, Chief Assistant to the United States Attorney Floyd Wheaton has assigned Eliot Ness to the case; Ness is visiting William Norbert in the hospital. Ness tells him the government has been building a case for 2 years against Luigi Renaldo, but William is too scared to talk. William has good reason to be scared: when he moves to a nice house in the suburbs with his wife and child, a couple of hoods in a car drive by and riddle the place with a chopper; their bullets narrowly miss William and his wife Millie.
William goes to Ness, who will place him in protective custody for several months; his wife and their little girl Jenny (about age 10) will stay with Millie's aunt in K.C., and also get police protection. For 10 years, William has kept facts and figures for Renaldo's rackets in his head; now he's telling it all to Ness and his men, who have 2 extra workers with adding machines filling rolls of papers with numbers. July 1934, Ness and his Untouchables arrest Renaldo. At a pre-trial hearing, to demonstrate William's ability to the judge, a clerk rattles off thousands of 5-digit numbers, which William adds up in his head (while 2 workers with adding machines frantically type in the numbers) and comes up with the correct total: 462,807,507.
Renaldo is indicted the same day; also out on bail the same day. In the following weeks, his shyster attorneys ask for and get one continuance after another. To keep William safe, Ness and his men shuttle William all over the country on trains; (we don't see any stock footage, just the words on the screen: Toledo, Albuquerque, Louisville, St. Louis, Kansas City, New Orleans, San Francisco, Atlanta). The constant travel wears William down; his only happiness is trips to Kansas City, which means he can briefly see his wife and kid.
Renaldo pulls a dirty trick...he has his hoods run Jenny over with a truck, breaking her legs. They figure it would smoke William out; it does, he rushes to the hospital. 2 big hoods are waiting for him; William has an adrenaline rush and punches them out. Then Ness and his men drive up; Ness shoots a hood, and his men arrest the other. Finally, in September, the trial is set to start...with a change of venue to Woodland, Illinois. Days before the trial, Ness has his men posted outside the courthouse; Youngfellow, with binoculars, takes the place of the flagpole sitter, Jackstraw Wilson, replete with clown costume.
On the day of the trial, Renaldo has 15-20 hoods imported, including Davey the Chopper from Detroit. In the courthouse, Renaldo figures William Norbert might be a no-show; but William is already safe inside the courthouse, disguised as a painter. Ness, sitting in the spectator section, stops Renaldo's man, Rienzi, who is reaching for his gun. Outside, Enrico Rossi gets the drop on a couple of hoods; Flaherty shoots one. Youngfellow is back in action on the ground, but hood Forenzi, from a boarding house window, then shoots Jackstraw Wilson on the flagpole. Ness and Rossman burst in and Ness shoots Forenzi, and they nab 2 other hoods including Davey the Chopper. The entire Renaldo mob is either dead or in custody. As for Renaldo, he was convicted on six of the twelve counts and received 20 years on each count but the sentence ran concurrent not consecutive. He would have been out in 12 years except for one little thing...he died in 1939 in prison.