April 11, 1932. Millionaire building contractor Thomas B. Randall is the target of a kidnapping; he is throwing a party right now. Intruding on his estate that night are: ex-bootlegger and now gang leader George "Blackie" Dallas, Pete Appleby (former torpedo for the Purple Gang), Marty Stokes (bank heist expert) and Jiggs (ex-heavyweight boxer and now strongarm man). The gang kills a security guard, and kidnaps Randall-- and they warn his family and guests not to call the police, or he gets it. They drive away. The mastermind behind this is George Dallas' ambitious wife, Lily Dallas; she pushed the small-time hood into committing ever more dangerous crimes-- with bigger takes and bigger risks. Lily is only recently out of prison, where she served 5 years for armed robbery-- Lily masterminded bank jobs which netted over $1-million, long since spent. Lily had a good teacher: Jack "Legs" Diamond.
Ness and his men are on the case. They find the abandoned getaway car; the crooks obviously changed cars. Ness talks to Mrs. Randall, he says her husband's only chance for survival is if she cooperates with the police. But it's too late, she has already dropped off the money in Lincoln Park. That night, Marty Stoke delivers the 300-grand ransom to the gang. Marty also comes on strong to Lily; she pretends to put him off, but when he stops his advances she encourages him by saying, "Marty, I hope I didn't let the air out of your tires." They've got the hots for each other. Thomas Randall, tied up in the next room, looks out the window and sees all but the last number of their car's license plate: C-211. George Dallas says they can release Randall-- he never got a good look at them, and he's too scared to go to the police; besides, he's got a wife and family. Cold-hearted Lily just sneers at his sentimentality and insists they rub Randall out-- in fact, she forces George to do it. Randall dies in a hail of bullets.
Randall's body is fished out of Lake Michigan. To buy time, Ness circulates a phony story to the Underworld (Capone's boys, etc.) that the money was marked. The coroner pays no attention to some scratches on Randall's arm, but Ness thinks Randall tried to give them a message: he makes out C-211, which is all but the last number of a license plate. There are 10 possible numbers for the last digit; checking with the Bureau of Motor Vehicles, Ness gets the names and addresses of 10 owners. Meanwhile, Lily Dallas hangs around her legit front: the "Fleur-de-Lis" Beauty Salon. That night, Lee Hobson is checking on the car with license plate C-2113. It belongs to Mrs. Murphy, (Lily's mother, but he doesn't know that). When Mrs. Murphy refuses to cooperate and acts suspicious, Lee Hobson phones Ness and they come back with a search warrant-- by that time, Mrs. Murphy has called George Dallas and they've flown the coop.
The car is still outside, though, and a search of the house yields a clue: some lipstick that's labelled "not for sale," which was distributed to beauty shop owners only. Ness meets with the manufacturer who passed out the samples-- in the office of manager Francois Simone, when Ness asks for a list of those owners who got the lipstick samples, Francois' sexy secretary Danielle says something in French to Eliot Ness that makes him blush.
Ness drops in on Lily Dallas at her beauty shop. He asks her about a Mrs. Murphy; she says Murphy was her maiden name, but her mother's dead. Lily claims to know nothing of the lipstick sample, either. Then Ness shells out 10 bucks to buy a compact, ostensibly for his wife; Lily sees right through this (Feds only make 30 bucks a week)-- she knows Ness wanted a sample of her fingerprints. Later, Lily gets on the phone and calls Marty Stoke; she says she wants to fence the money, run off with him and "deal the others out." Lily tells Marty, "We'll live it up like the world is gonna end in a week-- like we're on the tail-end of a firecracker!" Lily doesn't know her hubby George is overhearing her conversation. Meanwhile, Ness and his men run a make on Lily's fingerprints and find out all about her: her previous prison record, and that she was the power behind Jack "Legs" Diamond.
Ness' men stake out her beauty shop. Lily puts on her fur coat, which has the 300-grand stuffed in the lining. George and Lily drive off; in the car, she asks George for the name of the fence who will give them 50 cents on the dollar for the hot loot. George says that doesn't leave much divvied up 5 ways. Lily says it'll only be divvied up 2 ways; George, knowing about her and Marty, asks, "Who's the two?" Nevertheless, he drives her to Sal Figero the fence. But Sal says the marked loot is so hot, only someone with a death-wish would touch it. Lily tells George to blast Sal; when George balks, Lily grabs the chopper and blasts Sal herself.
At their hideout, they decide to rob a bank at 3 o'clock to get 100-Gs. Lily's going to run off with Marty; George says if she does, he'll never tell her where he hid her mother and her kid Arlene. Marty and George fight; Marty wins, George says they're at the Hotel Trafalger. Marty, Pete and Jiggs pull off the bank robbery, with the help of bank accountant Morgan Gates; but Marty pays Gates off-- with hot lead. Lily drives Marty, Pete and Jiggs over to the Hotel Trafalger; Ness and his men are waiting there. There's a shootout; Marty and Pete get gunned down, Jiggs surrenders. Lily goes up to see her mom and the kid, but George is waiting there for her, with a chopper; this time he doesn't balk, George mows her down. Then Ness shoots George. But with his last bit of strength George, laying on the floor next to Lily, reaches out to hold her hand.