District Attorney Downey tries unsuccessfully to convince parole board chairman Jed Cross that certain vicious criminals should not be released on parole. Meanwhile, cop killer "Slicker" Nixon and his men, recently paroled from prison where they were serving time for minor offenses, plot to start a new racket disguised as a legitimate business. Because store owner Mallory refuses to go along with the business, actually a "protection" racket, he is killed. Although Mallory's daughter Pat witnesses the crime, Nixon's airtight alibi keeps him from being convicted. Pat then decides to catch and kill Nixon herself while Nixon and some of his former prison mates work on further schemes. As she hides to listen to Nixon's plans, she is discovered by "Red" Herron, a member of the gang, who lets her go. After being evicted by her landlady, Pat wanders the streets carrying a gun concealed in her purse, then falls asleep among the hoboes. A kind policeman awakens her and takes her for some food at the police station, where she encounters Binnie Bell, a marijuana user who is friendly with Nixon's gang. Binnie convinces Pat to take a job at the Castle Inn, a gangster hangout. Downey, concerned for her safety, also goes undercover to infiltrate Nixon's gang, and soon discovers that Jed Cross has been meeting there with several criminals. Unknown to Pat, Nixon plans to murder Joe "Killer" Britt, who served time in prison for the murder that Nixon actually committed. Before Britt is killed, however, Nixon sees members of a rival gang arrive and realizes that Britt has joined them. As shooting starts, Red rushes Pat out of the way. Later, at the police station, all of the criminals are arrested except for Red, who turns out to be an undercover police lieutenant and the friend of the policeman killed by Nixon. With the mystery solved, Pat collapses into Downey's arms.