It's the early days of the F.B.I. - federal agents working for the Department of Justice. Though they have limited powers - they don't carry weapons and have to get local police approval for arrests - that doesn't stop fresh Law School grad Eddie Buchanan from joining up, and he encourages his former roommate James "Brick" Davis (James Cagney) to do so as well. Davis, having completed law school, wants to be an honest lawyer, not a shyster, despite his ties to mobster boss McKay. He's intent on doing so, until Buchanan is gunned down trying to arrest career criminal Danny Leggett. Davis soon joins the "G-Men" as they hunt down Leggett (soon-to-be Public Enemy Number One) and his cronies Collins and Durfee, who are engaged in a crime and murder spree from New York to the midwest.