The film is basically about the 2 classes of people in the industrial age, which are the planner and management who live on the ground in a futuristic mega city with luxurious skyscraper. Another class is the working class people that live in the underground. The city was build by Joh Frederson, who has a son named Freder, his life was all about luxurious, but one day that he was at the eternal garden, he saw a bunch of children with a woman who are a working class people. Freder was fascinated by them and when those children and a woman was being chased back down, Freder followed them down and had a chance to experience about the working class people's lives for the first time, that is not so pleasantly to please, when there is a machine that exploded and had killed many people.
After that he has confront his father about this issue, about the explosion and the working class people, but his father didn't listen to him. In which has made Freder going down in the underground and work taking shift between the working people.
In my opinion, this movie is trying to convey about the social crisis about those period of times between the working class and the owner, that the working class people are slave and working under the owner and that they are being treated badly. Somehow that the worker is like a machine that is working with a machine itself.