The Monster (1925) on Netflix

Milquetoast small-town store clerk Johnny Goodlittle is smitten with his boss's pretty daughter, Betty Watson, and is enrolled in a correspondence school course as an aspiring private investigator. After receiving his diploma, badge, and gun, he involves himself in a local missing person case. The mysterious disappearance occurred in the vicinity of an old dark house which once functioned as a mental sanitarium but is now apparently abandoned. When Betty and dapper Amos Rugg, Johnny's co-worker and rival for her affections, become trapped in the asylum, Johnny discovers that the asylum is not deserted but run by the eccentric Dr. Ziska and his bizarre staff and puts his correspondence school training to the test.