A gold mine in Arizona, that was formerly losing a lot of money, suddenly turns into a veritable money-making machine. However, the owner, instead of being happy about his now profitable business, insists to Charlie that something is fishy and that someone is out to murder him. Charlie and his "crew" travel to the mine, pretending to be tourists staying at a nearby dude ranch so as not to arouse suspicion, and discover that the owner may well be right--it looks like the mine is being used as a cover for criminal activities, and that someone is indeed out to murder him.
One of the better outings for Roland Winters as our unflappable Hawaiian sleuth. This time he has to fathom a mysterious conspiracy that has turned a loss-making hole in the ground into a worryingly profitable mine - and the owner feels certain that he is about to get his comeuppance. We see quite a bit more of No. 2 son "Tommy" and "Birmingham Brown" in this one - and they add quite a bit to his actual quite clever little Monogram mystery.