Samavia is a small kingdom somewhere in Central Europe which has been torn by internal strife for centuries as various factions squabble over the throne, left vacant after the last king of the old royal house was assassinated and his son, the noble Prince Ivor, disappeared. Legend has it that Prince Ivor survived the assassins' attack and went into hiding somewhere, and that one day his heir will return at the head of a secretly-gathered army, sweep away the squabbling factions, and restore the country to rights.
Marco Loristan is a young Samavian who has never seen Samavia; his father is a displaced Samavian patriot who travels from country to country attempting to secure aid for his stricken homeland. At the beginning of the novel, they come to live in London, where Marco befriends a group of street kids led by a crippled boy called "the Rat", who invents games for them in which they are soldiers preparing to fight for a noble cause. Inspired by the plight of Samavia and the legend of the Lost Prince, the Rat turns his mind to how he and his friends might be of use, and comes up with a plan whereby boys like himself and Marco might serve to carry secret messages without arousing suspicion.
Unexpectedly, he is offered a chance to put the plan into practice for real: the network of patriots Marco's father is part of have decided the time is ripe for something to be done about Samavia, and there is a need for certain secret messages to be carried. As Marco and the Rat learn more of what is going on, it becomes apparent that there's more truth to the legend of the Lost Prince than one might have believed.
This novel provides examples of:
- Abusive Parents: The Rat's father.
- Alcoholic Parent: The Rat's father is alcoholic and abusive.
- Clap Your Hands If You Believe: Part of the philosophy that Marco learned from his father, who learned it from a Hermit Guru, holds that sufficiently enlightened people can affect reality through their thoughts: "Meditate only upon the wish of thy heart, seeing first that it can injure no man and is not ignoble. Then will it take earthly form and draw near to thee." Marco uses this several times through the novel, with apparent success.
- Genius Cripple: The Rat, to an extent; though physically disabled, he has a keen mind, and in particular has a grasp of strategy and tactics better than many adult men.
- The Good King: Legend depicts the kings of the old royal house as this. The secret heir who is crowned at the end of the novel shows all the signs of being one.
- Hermit Guru: Marco's father met one on a mountain in India years ago who gave him renewed hope at a particularly dark point in his life.
- Identical Grandson: While not identical (if only because he's a different age), the Lost Prince's present-day descendant bears a striking resemblance to the surviving portrait of his distinguished ancestor considering the number of intervening generations. Many Samavians choose to take this as a sign that the Lost Prince has been reincarnated to return in person, which makes as much sense as any other explanation.
- King Incognito: When Marco and the Rat are introduced to the new King of Samavia at the end, it turns out they've met him before without knowing that he was the Lost Prince's secret descendent.
- Old Retainer: Loristan's servant, Lazarus.
- Only Known by Their Nickname: The Rat. His real name, Jem Ratcliffe, is mentioned once when he first introduces himself to Marco, and then never again.
- Rightful King Returns: Legend says that the Lost Prince's descendents live on secretly, gathering support, and will one day return to put the country to rights — and it all turns out to be pretty much true.
- Ruritania: Samavia is one of the last hurrahs of the pre-WWI romantic Ruritania. It's somewhere on the far side of Austria-Hungary from England, but the one time its precise location is given it is expressed in terms of the countries Samavia shares borders with, all of which are just as fictional as it is.
- Single Line of Descent: The Lost Prince's descendants apparently managed to maintain one of these, complete with a lack of any watering-down issues.
- Succession Crisis: Samavia has been suffering one of these ever since the Lost Prince disappeared; in the absence of an heir of the old royal house, two rival families have been taking turns kicking each other off the throne.