„Un peuple et son roi" crosses the destinies of the men and women of the population, and those of historical figures. Their meeting place is the newly founded National Assembly. At the heart of the story lie the fate of the king and the birth of the French Republic.

Cesar is a young schoolboy living in Paris with his family. Their life is ordinary, but Cesar wants more excitement (which he creates, in one instance, by claiming to his teachers that his father has been arrested). During the school holidays, Cesar and his friend Sarah decide to help their mutual friend, Morgan, find his father who supposedly lives in London. Each one tells their parents that they're staying with the other two, and together they sneak out to begin their search.

A detective captain learns that a mummy in an SS uniform has just been discovered, walled up in an old building undergoing renovation in Colmar...