XIII is a cel-shaded first-person shooter based on the popular Belgian cult comic created by Jean Van Hamme and illustrated by William Vance. The game is loosely based on the first five comics. The main theme of the game is based on Robert Ludlum's book The Bourne Identity, and features a man who is suffering from amnesia and quite possibly involved in the recent assassination of the President of the United States. The hero knows almost nothing except that he must be important, since everyone is trying to kill him. Gradually uncovering the mystery, the hero, learns more about his secret agent past and his curious name: XIII.

The player unravels clues, trying to solve a conspiracy set in an environment that breathes a comic book ambiance. Footsteps and shots are visualized in text and actions: for example, head shots are shown in a series of still images. Aside from that, the game has all the typical FPS elements: both stealth and action, more than fifteen weapons (shotguns, crossbows, rifles, ...), interaction with environmental objects and plenty of enemies in levels spread all over the world. During the game, the player can learn a number of skills such as silent walking, lockpicking and dual weapons handling.