The old iron-clad Meridian steams south on her last journey under the British flag, on route to her new home in the Chilean Navy. Using the transfer of the ship as cover, Halfhyde and Watkiss are on a covert operation to protect British interests in South America from the encroaching Germans. Soon Halfhyde has an added mission: he must help a detective from the Metropolitan Police track down and intercept a traitorous civil servant who has escaped from prison and is on the run for South America.