Through a loophole in the law, it's possible to acquire a sub-machine gun without a police permit and soon Chicago's gangsters are killing each other in record numbers. The Capone gang in particular is after Bugs Moran and his north side operation. The dispute eventually leads to the infamous St. Valentine's Day massacre. Public reaction soon leads to a change in the law and Eliot Ness and the Untouchables are confiscating all the weapons they can lay their hands on. It doesn't stop the traffic in illegal arms however and when Frank Nitti, in charge of the Capone gang with big Al away in Florida, is approached by someone who proposes to make machine guns for him from scratch, he jumps at the opportunity. He also sets up a trap to try and get rid of Moran once and for all.
—garykmcd