When counterfeiters pass a twenty-dollar bill to Sydney Foster at Dalton City, the Foster twins begin another one of their exciting adventures. They are determined to catch the crooks and see that they are punished for breaking federeal law and, in this case leaving Sydney short of money to pay for her new metal detector and microscope. Before the Foster twins are able to do what they set out to do they are captured and left on a snake and alligator infested island and set adrift in their own boat to go aver a giant waterfall to the rocks below. They know that even if they survive this challenge they will eventually have a life or death battle with two unbalanced killers if Chief Cupp doesn't arrive at the old country house in time This is the twins' most exciting adventure yet