Things start off innocently enough as Ken's father, Richard Holt comes to stay with the Allen's for the Christmas holiday. Naturally he brings gifts, one of which is for Mom Allen, an ornate iron box. This is where the mystery originates. Strange unexplained events occur at the Allen and Holt residences. There are prowlers who may be connected to Mr. Holt's investigation of the counterfeiting of ten dollar bills.
When Ken and Sandy bring the iron box to Sam Morris's jewelry store in Brentwood for a minor repair, the mystery deepens. A fire is set there and it becomes apparent that someone wants that box, but why ? How is the enigmatic Amos Barrack involved ? The boys sleuthing takes them to the the docks of New York where Barrack is encountered again. Twice Ken and Sandy are held captive by the gang and while failing to escape the second time, do manage to leave clues for others to pursue.
The boys are transported out to sea on a barge, where by their wits alone, are able to escape the immediate clutches of the desperate villains. Still their plight is dire, as a storm threatens to sink the barge they are still on. It is only after the boys are rescued (due to the clues they had cleverly left behind at the warehouse on the docks), that the true nature of the gangs criminal activity is revealed and the part played by the iron box is at last exposed.
