Royal Flash, from the Flashman Papers, 1842-3 and 1847-8 is a sequel to George MacDonald Fraser's Flash, a book which started the chronicles of Captain Harry Flashman. The book is written as part of a war hero's memoirs and misadventures.

Captain Harry Flashman, also known as Flash or Flashy, is a member of the Horse Guards, part of the British military. Flash is a highly decorated and admired soldier despite his relatively low rank. Flash does not much care about rank, however. Instead, the man prefers to focus on what his notoriety can do for him, in the way of invitations and forgiveness.

The book opens with Flash and his friend Speedicut going to a new club—one that still allows all of the things slowly being banned in Victorian England. In the midst of the evening, the club is raided. Flash and Speedicut manage to make a clever get away. A boy from the club points out the two men to the police and a chase ensues. Speedicut stays to fend off the Bobbies while Flash makes a run for it, hindered by an injured leg. Flash hides in a carriage and is discovered by Rosanna James. Rosanna is a saucy Irish woman married to a fellow British soldier. Although it angers Rosanna's companion, a Prussian soldier named Otto, Rosanna defends Flash to the police and then allows him to go home with her. The two embark on a wild and turbulent affair. Although it ends with Flash running for his life, the two meet again.

Flash goes about his life and while on a hunting expedition, runs into Otto, who loathes him. Otto turns out to be a baron and is even more obnoxious, pompous and condescending than the night he and Flash met. Otto does his best to humiliate Flash and win every competition, no matter what the price. The trip does not go well, but Otto and Flash pretend to part on civil terms.

Flash sees Rosanna again and learns that she is now using an alias, Donna Lola Montez. Flash sees to it that Rosanna is unmasked during a dance performance and Rosanna leaves London, swearing revenge.

Much later, Flash gets a letter from a mysterious countess begging for his audience. Flash uncovers that the woman is Rosanna, now referred to as Lola, the Countess de Landsfield. Against Flash's better judgment, he accepts Lola's invitation.

Flash become embroiled in an evil and complicated plot involving Otto and the impersonation of the Danish prince Carl Gustaf. The scheme takes Flash to the Duchy of Strackenz where he marries a duchess, nearly dies on a number of occasions, kills a man, and steals a fortune in jewels. Lola resurfaces and she and Flash sleep together again. When Flash awakes, he finds that Lola has robbed him.

Flash finally makes his way home to London and life resumes as normal.