City of endless night

The year is 2041. Since the end of the First World War, Berlin has been an enormous subterranean ciy, home to 300 million citizens who have never seen the sun and presided over by the autocratic Hohenzollern dynasty.

Lyman De Forrest, and American chemist, discovers a way of neutralising Berlin's defences and, assuming the identity of a dead German, enters the city to discover its hidden truths.