Based on

Movies like Parole Racket (1937)

Following a long investigation, Police Detective-Lieutenant Anthony "Tony" Roberts becomes convinced that there is some powerful unknown-master-mind leader behind the gangs of crooks who are terrorizing the city, and it has something to do with paroled convicts. He sets up a scheme with his supervisor to have himself shamed by being demoted to a patrolman and then caught in the act of receiving bribe-money from a racketeer. He is convicted and sent to prison, and the manner in which his parole is arranged leads him, dangerously, to the secret-head of the gang.