Clark, Jimmy, and their new friend Pug Flanagan have breakfast as their ship sets sail to steam back to the United States. The boys worry over a threatening message they recieved, but Clark tries to play it down. Still, when more threatening notes arrive, he takes the concern to the ship captain. The boys aren’t buying Clarks attempt to play it down, and they each do their own investigating.
Snooping in the cargo hold, Pug and Jimmy learn what the ship is really hauling. But more troubles arise for the boys.