This is the first of a two part Untouchables story where Robert Stack and his picked crew of incorruptible federal agents get involved in protecting the Mayor of Chicago one Anton J. Cermak elected in 1931 against the Capone mob's friend in City Hall William H. Thompson. That was a seminal election in Chicago's history, Democrat Cermak defeating Thompson, no Republican has won since.
Organized crime had a friend in Thompson, not that Cermak was anything like a boy scout, but Thompson was once chosen as the worst big city mayor by a poll of urban historians. Hard to argue with that. And the mob missed the friendly back door to Chicago's City Hall they once had.
It's another one of those historical legends that the real intended target in Miami was Cermak instead of President-elect Roosevelt. This episode takes no position on that, this is just The Untouchables on another case.
Robert Middleton fits my conception of Cermak from all I've read about him. On a parallel track in the plot we see Giuseppe Zangara, Italian immigrant and anarchist, an angry man with stomach problems and Joe Mantell fits what I've read of Zangara. It was true he had originally planned to kill Herbert Hoover, but as a passerby told him on Election Day Night, the Democrats did it for him. After that he sets his deranged sights on FDR.
The two parts were later released as a film for the big screen entitled Gun Of Zangara.