After the Chief Deputy of the Department of Weights and Measures is nearly killed in a car accident engineered by corrupt politician Marty Cavanaugh, he enlists ex-boxer Johnny Cave (Cagney) to take over his position. As the new leader, Johnny reiterates to his team the importance of their department and warns them that corruption is an ongoing hazard.
Johnny then goes out into the field with his naive partner, Patrick James "Aloysius" Haley, investigating merchants who are accused of using faulty measures and cheating the public. He ends up fining a market for adding lead weights to stewing chickens and fining a gas station for routinely shortchanging its customers. In each case, the merchants try to bribe Johnny in exchange for ignoring their corrupt practices, but he adamantly refuses. Meanwhile, Johnny's fiancee, Janet Henry, criticizes him for being constantly hardheaded in his indefatigable pursuit of fighting corruption.
Later on, Cavanaugh offers Johnny a cushy job with his organization in exchange for turning a blind eye to his citywide racket. After he refuses, Johnny is framed for both drunk driving and a robbery, but is then "exonerated" by an ornery Cavanaugh, implying that he can make or break him. Afterwards, the mayor, a puppet for Cavanaugh, offers Johnny a high paying job, but once again he refuses.
When Johnny learns that Janet's boss, Abel Canning, has been swindling a local orphanage by sending them half-shipments of food but charging them full-price, he declares that he's going to expose him for the criminal that he is. Once he realizes that Canning is in an alliance with both the mayor and Cavanaugh, Johnny releases the orphanage story to the newspapers, which angers his fiancee and eventually leads to her breaking their engagement.
As Johnny prepares for the case against Canning, Cavanaugh hires a thug, ex-wrestler Joe Burton, to attack and steal the evidence from him. However, instead of turning over the evidence to Canning, Burton decides to blackmail him for $5,000. While at a big cocktail party, Canning gives Burton a $5,000 check in exchange for a key to his apartment where the evidence is hid. After Johnny notices Canning at the party with a skeleton key, he spots Burton exiting a side room. Johnny goes over to Burton and punches him in the face, then removes what he thinks is the stolen evidence from his jacket pocket, but instead discovers a check written by Canning. He then realizes that Canning is on his way to retrieve the evidence in Burton's apartment.
Meanwhile, at the apartment, Canning and Cavanaugh locate the evidence hidden behind some wallpaper in a closet. They are about to burn the papers when Johnny arrives just in time, preventing them from destroying the evidence. Then, thanks to a tip by Janet, the police arrive moments later and arrest the two men. Later on, with Johnny and Janet's engagement back on, he presents her with a ring that he got on the "installment plan," even though he knows it's a racket.James Cagney as Johnny Cave
Mae Clarke as Janet Henry
James Burke as Patrick James Aloysius Haley
Edward Brophy as Pete Reilly
Henry Kolker as Abel Canning
Bernadene Hayes as Hazel Scott
Edward McNamara as Captain Pat Hanlon
Robert Gleckler as Marty Cavanaugh
Joe Sawyer as Joe Burton
Mary Gordon as Mrs. Ogilvie
Great Guy was the first of two films James Cagney made for independent film company Grand National Pictures, after he successfully broke his contract with Warner Brothers. After completing his second film, the musical Something to Sing About (which was not a financial success), Cagney returned to Warners.
The technical adviser for the film was Charles M. Fuller, who was the Los Angeles County Sealer of Weights and Measures. The story was based on several written pieces that appeared in The Saturday Evening Post in 1933 and 1934, written by James Edward Grant. Early press releases for Great Guy list Henry McCarty, Harry Ruskin and Horace McCoy as contributing writers for the film, but later on, McCoy was dropped from credits, and Henry Johnson was added as a co-writer.