Eliot Ness and The Untouchables are working in St. Louis to take down a syndicate kingpin played by David Brian. He and his chief enforcer Leo Gordon have brought a certain amount of order to crime in that city with Brian a respectable front and Gordon supplying some really mean muscle for the dirty work.
One of the hoodlums they brought into their operation pulls a mail truck robbery that nets him cash and negotiable securities. Brian and Gordon cut themselves in and then quietly eliminate all who could finger them. Of course Robert Stack does catch up with them because of a talkative landlady played by Lillian Bronson.
Anthony George joined the cast of The Untouchables in this episode and he's got a long standing personal score with Brian dating back from when he was not so respectable.
I think the writers forgot in this episode they were dealing with the Depression and anything to do with stockbrokers was not a paying proposition. Nobody would be robbing securities because of the Stock Market Crash.
Gordon and Brian are a pair of nasty villains though and you love seeing them get taken down.