Officer O'Brien stars future "Hopalong Cassidy" William Boyd in the title role. Though highly respected by his fellow policemen, O'Brien is continually humiliated by the criminal activities of his reprobate father (Ernest Torrence). The old man finally makes amends for his past sins by killing the gangster boss. Still, O'Brien is determined to quit the force and start life all over again -- a determination that lasts, oh, about two minutes. To avoid confusion with stage actor William Boyd, the star of Officer O'Brien was billed as William "Screen" Boyd.