King Rat is a competitive, diceless roleplaying game inspired by James Clavell's novel King Rat. In the game, the PCs ("King Rats", as opposed to plain "Rats") try to carve out an empire while trapped within a closed community or organization (called "The Nest") like a POW camp or a megacorporation. They all seek "The Thing", which may be tangible like a weapon or intangible like a scrap of information, that will grant the needed power to take control.

Scenes normally alternate between players, but there is a bidding mechanic that allows a player to usurp the order by wagering resources. Resources are also used within a scene to accomplish goals, or frustrate other's attempts to achieve their goals. Relationships are important; resources can be used to develop friends, but the flagrant expenditure of resources also increases the Hatred meter, which generates Enemies. The winner becomes just "The King", and gets to narrate the last scene, or "Exodus", during which the closed environment crumbles.