“I mention it,” Eddie’s father said, “only because, if it’s true, the tubes which have been picked up off the sand bar are already delivered. In that case, your blueprints and my radioisotope are no longer secrets. If not, however, both still must be on the submarine. No sub could shuttle back and forth to a foreign shore fast enough to make delivery and get back within a week’s time. This is only a guess, but they may lie a few miles offshore during the week as a safety measure and to conserve fuel. They come in and surface just outside the bay each Saturday, under cover of darkness. When they have everything they’re after, they’ll head home. Since they already have sufficient samples of the isotope, my guess is that they are now after the final blueprints. The small samples of the isotope are now used only as tracers to help locate the submerged cylinders.”