The Unknown was the last of three Columbia programmers based on the legendary radio series I Love a Mystery. Carried over from the radio version are Jim Bannon and Barton Yarborough as Jack Packard and Doc Long, adventurers for hire. The plot concerns the efforts of amnesiac Nina Arnold (Miss Jeff Donnell) to claim her rightful share of her domineering grandmother's legacy. Someone is trying to murder Nina, and that someone may very well be her emotionally unstable mother Rachel Martin (Karen Morley). But with Jack and Doc on the case, Nina has nothing to worry about-or does she?