A San Francisco airplane bound for South America crashes, and among the scorched debris is found a shrunken native human head, neatly packaged. The perplexed police contact a local anthropology museum about this unclaimed piece of grisly baggage, where they intersect with Jack and Doc, two private eyes, called there to meet a mysterious woman who had a case for them and wanted to meet in private.

Chan is faced with suspects in a stolen atomic bomb formula case, that are being killed with bullets that are not fired from a gun.

In San Francisco, detective partners Jack Packard and Doc Long are hired by socialite Jefferson Monk who believes someone is following him with the aim to kill him.

"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.