The two, and only real characters in Invisible Cities are the famous Portuguese explorer Marco Polo and the Mongol emperor Kublai Khan. Though both are historical figures from the 13th century, the novel is a fictional account of Polo's descriptions of his travels through Khan's kingdom.

Polo is not a necessarily a reliable narrator and the author states from the beginning that

Kublai Khan does not necessarily believe everything Marco says when he describes the cities visited on his expedition.

He does, however, enjoy listening to his tales. One reason the author give for this is

In the lives of emperors there is a moment . . . that this empire . . . which had seemed to us the sum of all wonders, is an endless formless ruin . . . Only in Marco's Polo's account was Kublai Khan able to discern, through the walls and towers destined to crumble, the tracery of a pattern so subtle that it could escape the termites gnawing.

At first Polo can only describe his adventures through the use of pantomime,

one city was depicted by the leap of a fish escaping the cormorant's beak to fall into a net; another city by a naked man running through fire unscorched.

but as his knowledge of Tartar improves he starts to express his travels through both language and movement.

The emperor is not a patient listener and it soon becomes apparent that his reason for hearing the stories is to "possess my empire."

At one point he states:

If each city is like a game of chess, the day when I have learned the rules, I shall finally possess my empire, even if I shall never succeed in knowing all the cities it contains.