On a tramp steamer off Central America are Mr. and Mrs. Sheppard, five prisoners en route to a Nicaraguan prison, and Corbett, an American carrying money for a Honduran counter-revolution. Denied permission to land in Honduras, Corbett releases the prisoners and with their aid hijacks the ship. They land, taking the wealthy Sheppards as hostages, and start the arduous trip upriver to Corbett's rendezvous, meeting jungle hazards