"The Unknown" was the final entry in Columbia’s I Love A Mystery series. A woman hires two detectives to keep her alive long enough to claim her inheritance.
| Actor | Role |
|---|---|
 Karen MorleyRachel Martin | Rachel Martin |
 Jim BannonJack Packard | Jack Packard |
 Jeff DonnellNina Arnold | Nina Arnold |
 Mark RobertsReed Cawthorne (as Robert Scott) | Reed Cawthorne (as Robert Scott) |
 Robert WilcoxRichard Arnold | Richard Arnold |
 Barton YarboroughDoc Long | Doc Long |
 James BellEdward Martin | Edward Martin |
 Wilton GraffRalph Martin | Ralph Martin |
 Helen FreemanPhoebe Martin | Phoebe Martin |
 Robert KellardJames Wetherford (as Robert Stevens) | James Wetherford (as Robert Stevens) |
| JJ. Louis JohnsonJoshua (uncredited) | Joshua (uncredited) |