During the blistering summer of 1931, Ness and his men are working tirelessly against both the illicit whiskey and narcotics that are flooding the city. One morning, a despondent Capt. Jim Johnson visits Ness in his office. Capt. Johnson had been on a raid that netted 50 dope addicts, one of them was his son Buzz. Ness talks to Buzz behind bars. Buzz cooperates, and says he got the powder from a pusher named Peepers. Ness gets Buz released. Peepers, meanwhile, is awaiting a big shipment of narcotics from Sal Rudin, alias "Mr. Big."
Agent Lee Hobson traces Peepers to where he's staying, which is the Viking Hotel. Ness and Lee see Peepers carrying a small case of dope. Peepers sees the two G-men and fires his gun at them. Ness shoots back killing him. Searching Peepers' apartment, they find out that Artie Tresh, a string bass player working at the Club 88, also lives there. They go to the Club 88, and meet the owner, Sal Rudin. When Ness says he wants to talk to Artie Tresh, Rudin tells him that he left suddenly yesterday so he fired him. Tresh apparently is headed for New Orleans. When Ness asks why he fired him, Rudin tells Ness candidly, "I found out he was playin' more than the bass...he was playin' around with my wife."
August 11, 1931. On a train to New Orleans, Artie Tresh, a heavyset guy who looks nothing like Ness, gets shot in his compartment. Ness takes the body back to a Chicago morgue for positive identification, which is supplied by Howl Temple, a trumpet player who knew Tresh from Baltimore. Mr. Temple, a fine gentleman and a great jazz musician, plays his horn for his pal one last time as a memorial. Then Eliot Ness tells Lee Hobson that he will take Tresh's place and will impersonate The Jazz Man. Ness goes to New Orleans. Down in the Big Easy, Ness, wearing a bow tie and a straw hat meets booking agent Russ Bogan. Ness tells him he is Artie Tresh but Bogan is suspicious.
Ness checks out a violin-making shop owned by Karl Reising. That night, Ness goes to where he is booked later in the week, the Purple Elephant, a Dixie Land jazz club on Bourbon Street. Ness introduces himself to the club owner as Artie Tresh. Ness meets Hal, the clarinet player who leads the combo, and the lovely singer Lorna. Hal thinks Tresh is honing in on his girl, and picks a fight with him. But Ness only wants Lorna as a contact to get some junk.
Bogan tells Lorna to call up Tresh, cause he wants to "give it to him" but not talking about dope. He wants his hit-man to give him a few slugs from a high-powered rifle. Ness then gets a couple of welcome visitors; Lee Hobson and Enrico Rossi down from Chicago. As Ness walks down the street to meet Lorna, the hit-man gets a bead on Ness but Lorna, who's gone sweet on Tresh, pushes the hit-man, and he misses. Bogan throws her down the stairs, giving her a fractured skull.
Rudin is now down in New Orleans meeting with Bogan when one of their men recognizes Rossi as a fed from Chicago, prompting the hoods to exit and head for Reising's shop. Ness and Lee Hobson have the dope-smuggling system figured out. Ness stomps on Tresh's upright bass where cleverly concealed inside is a container of narcotics. They figure only a master violin maker could construct that, and they, too, head for Reising's shop. Ness and Hobson catch the crooks in the basement and shoot a henchman, while Rudin and Bogan surrender. Sal Rudin will face charges of dope smuggling. As for Russ Bogan, he was tried as an accessory to the murder of Artie Tresh and found guilty. On the night of January 8, 1934, he took his final steps toward his very last booking...the electric chair, in the death house at Joliet.