Johnny Moon is a mob boss who controls everything from politicians to a profitable women's reformatory he secretly runs. He has ruined many lives, including that of Helen Martin, a teacher, and her sister Jean.
A political reformer, Frank Donovan, is able to persuade Helen to go undercover behind bars, posing as a teacher of the inmates, to unearth evidence that will prove Moon's abuse of the incarcerated women. She ultimately succeeds, but not before placing her life in grave danger.
