April 1798 and Commander Devenish is unexpectedly ordered away on a mission to the French Brittany coast, to rescue a British agent from tenuously held rebel territory.
They are accompanied by a detachment of army scouts, but their commanding officer gives cause for concern and his son places the whole expedition in jeopardy. Meanwhile Devenish discovers that he is being shadowed by a former smuggling colleague, intent on a mission of his own.