During a rebellion of prisoners at the San Quentin State Prison, Boston Blackie makes a lightning escape aided by Mary McGinn while chased prison guards. Mary is a school girl, unaware that her brothers are crooks. She is expelled from school. Blackie rejoins his gang and takes refuge in Mother McGinn's house, where he again meets Mary. She devotes herself to making Blackie go straight and wins her point.
| Actor | Role |
|---|---|
 Colleen MooreMary McGinn | Mary McGinn |
 Forrest StanleyBoston Blackie | Boston Blackie |
 Margaret SeddonMother McGinn | Mother McGinn |
 Hobart BosworthWarden | Warden |
 George CooperTravel | Travel |
 Eddie PhillipsThe 'Glad Rag' Kid | The 'Glad Rag' Kid |
 Wade BotelerDetective O'Leary | Detective O'Leary |
| TTom BatesSandy | Sandy |
 Carmelita GeraghtyEthel Grayson | Ethel Grayson |
 Maxine Elliott HicksNot listed | Not listed |