Having made an enemy of Otto von Bismarck a few years earlier, Flashman now finds himself compelled by Bismarck to impersonate a Danish prince in a German duchy, taking his place in a marriage to the duchess. Flashman is a doppelgänger for Carl Gustaf and with his talent for languages, he’s able to pull it off. At first he believes that Carl Gustaf is recuperating from an embarrassing case of the pox, and he settles into enjoying his role. Then he learns that Bismarck is trying to trigger the Schleswig-Holstein Question and is holding the prince a prisoner. Flashy, poltroon that he is, would rather flee, but Danish partisans compel him to rescue the prince.
If this all sounds like The Prisoner of Zenda, that’s because Flashman claims that he told the story to the author of that book. Royal Flash is the sequel to Flashman.