May 8, 1931. The special U.S. mail train, coming from Rock Island, is stopped by emergency flares on the tracks in Hillsdale, Illinois. Gangsters wearing Army gas masks lob tear gas grenades into the train. They then shoot a postal clerk and make off with a million dollars in negotiable securities. Three months later, "Tough" Tommy Karpeles, former big-time Chicago hoodlum, is arrested for complicity in the case. His two accomplices were not caught. With his previous record as an ex-con, who had done five years for robbery, he was found guilty and sentenced to life in the state pen. Since crimes involving the U.S. Postal Service are a federal offense, Eliot Ness and his Untouchables are on the case. Ness thinks Karpeles is innocent and the thought that the real killer is still on the loose bothers Ness so he visits Karpeles in prison.
Karpeles tells Ness he was nailed on a bum rap. Karpeles had 3 witnesses who testified he was at Mendy's bar that night; his daughter works there. Ness thinks it's odd that he based his defense on the testimony of three barflies who weren't believed, but his own daughter didn't testify in his defense. Tuesday, September 8, 1931, Albert Miris (once known as Arnie "The Wolf" Mendoza) goes to a movie theatre he owns; upstairs, Jake the projectionist tells him that "talkies are just a fad." In his private office, Miris meets with the 2 accomplices who pulled off the mail robbery: Collier, and Fritz Herrling who shot the mail clerk. Since Karpeles is in prison, Fritz wants his share of the million bucks, he wants to buy his main squeeze a fur coat. Miris tells him the securities are still too hot to liquidate; Miris is also angry that Fritz killed someone during the robbery. He gives Fritz a few bucks.
Ness tracks down Sally Karpeles, who insists on going by the name Miss Karr. She knows her father was at Mendy's bar the night of the crime, but due to his past criminal life, and his past brutalities, she won't lift a finger to help him. Ironically, Ness gets a lucky break in the case-- Fritz, while trying to steal a fur coat for his moll, took 3 slugs. Ness talks to him in the hospital before he dies; Fritz tells him Miris was behind the Hillsdale job.
Sally Karpeles loves Milton, a poor but honest guy; unfortunately, she's also hanging around with a shiftless Lothario named George Varsey. Sally hates her old man; but Karpeles loves his daughter-- when Ness tells him she is hanging around with a shiftless bum with a criminal record, Karpeles cooperates with Ness. On Miris' orders, Collier kidnaps Sally. Ness and his men arrest George and give him the 3rd degree; George says that Miris will fence the securities with Oscar. Only one person knows who Oscar is, and where his hideout is, so Ness goes to Karpeles in prison. Karpeles says Oscar has a summer place on the lake shore, but he'd have to take Ness there in person. Letting a lifer out of prison is unheard of, but the prison warden agrees.
After midnight, Ness and his men drive out to the summer place in 2 cars; in the back-seat of Ness' car is Karpeles, handcuffed to an armed prison guard. On hearing his daughter is being held hostage inside, Karpeles knocks out the guard, and takes his gun. Inside, Collier shoots Karpeles; he shoots Collier. It's only when her father is dead, that Sally realizes she loves her father. Lee Hobson gets the drop on Mendoza; the 2 killers are now dead and Mendoza is going to prison. As for Tommy Karpeles, he died as he lived but a man can play many roles throughout the course of his life and Karpeles played the role of loving father until the end.