The movie is symbolic of Tokyo in a post world war environment, which is obvious from the first few minutes. Note, though, how it starts with a massive nuclear explosion, forcing Japan to start over and rebuild. Over the next thirty years, Tokyo becomes a hub of technological advancement, and a breeding ground for new businesses and capitalist opportunities. In the process, the people get more greedy, the gap between rich and poor becomes bigger, and the government becomes full of self interest as opposed to service of the people.
The film is a kind of satirical way to look at Japan after the actual nuclear explosions of Hiroshima and Nagasaki. It's become a black hole, closing in on itself with all the greed and selfishness from the moves forward Japan made that were almost kind of forced on them because of the results of the Second World War.