City of Endless Night

It's hundreds of years after World War I. A World State has been established after the successful dominance of the League of Nations, and it governs the globe in relative peace and harmony, save for one spot: Germany. By the use of a ray weapon to maintain its airspace, Germany has refused to submit to the rule of the League, and has tunneled down. The city of Berlin is now a gigantic subterranean metropolis shrouded in mystery. The World State can do little more than bomb it for political show. Until one day a chemist discovers a way to neutralize some poisonous gas used by the Germans as point-defense, and decides to launch an impromptu attack on them. He manages to swap his identity with one of the dead Germans, and penetrates the city to find out a horrible truth. This Germany is an eugenic state of the highest order, with workers bred in castes and segregated by levels. Can he escape and bring light to this City?