As he arrives in Lutha, King Leopold has just escaped from his ten years' imprisonment at the hands of his scheming uncle, Prince Peter of Blentz.

Barney meets and falls in love with Princess Emma Von Der Tann, Leopold's promised bride, and then becomes intimately involved in Luthan affairs, working to help the king and ultimately allowing himself to be proclaimed as king while impersonating Leopold to prevent Prince Peter from seizing the throne.

The real king Leopold, who has been his antagonist throughout the second part of the novel, is mistaken for Barney and killed by one of Prince Peter's henchmen.

Unlike Anthony Hope's Ruritanian novels, The Mad King is set in a real world situation.

Lutha's circumstances mirror those of many Balkan nations forced to choose sides in the opening phase of the First World War.

When cave man Nu is released from suspended animation, Victoria discovers that she is the reincarnation of his great love Nat-ul.