Set on board a 25 mile long colony ship that set out from earth an unknown time ago. The ship suffered damage a couple of centuries ago and the occupants were awoken by automated processes and in the meantime several mutinies and machinery breakdowns have left the populace in a primitive and tribal state.
The descendants of the original occupants are now gathered together in several towns and villages spread around the ship in different compartments. People from towns in the engineering and engine sections have begun to suffer debilitating mutations and are shunned as lepers. Large settlements have also grown up around other areas like the armoury, warehouses and kitchens. National identity has given way to racial identity since no one really remembers anything about earth or the countries and their conflicts - tribal identity supercedes all of them anyway.
The ship is controlled from the city based around the bridge which is overseen by the Mandarin - a descendant of the leader of the last victorious mutiny.
Probably already been done (as with so many of my ideas) but what the hey.