The idea isn't new. That humankind is in mortal danger from some form of great disaster and have to flee to an underground region for safety. It's the emotion of fear in the entrapment of darkness that is developed to a certain degree and fresh here.
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The last surviving people on earth have only seen the underground city the original builders created more than 200 years ago. They have no knowledge about where they are in relation to the planet and their only source of light is generated by electricity which has been increasingly faulty as the years went by. As the blackouts get more frequent and the time spent waiting in the darkness became more and more unbearable, the quintessential questions of survival are revived and we see how the old, the young, the jaded, and those that are stuck in an endless monotony without any desire or motive to change - we see how they handle the dire situation, internally and externally. Then there are those who cling on to a comatosed yet happy world of song and religious togetherness. They seem outside of the mess, in an alternate state, but instead of being truly delivered from the material aspect of life, they simply appear zombiefied and entirely out of point. There is insanity in their total lack of coherence for the reality that is unfolding sturdily into the minds of all and the depth of the fear they are showing (by not showing) must be so immeasureable that they have to escape into such profound opposite to extremity.
It is the two protagonists that keep humankind in check. They are focused on finding a way out of the eternal condemnation to darkness and as they keep on moving with hearts aflamed with that desire, they eventually solved the seemingly unsolveable problem. And these are strong personalities, diffusing a father who is jaded yet brilliant and imposes his fears & negativity unto his son. The love for his father, the respect for their similar brand of genius, the regret for his loss in contribution to the society and the frustration with his insecurities and lost of hope - these influences could have been a distraction for that bright boy alight with a beautiful fire inside him, but he kept his head and his dream and pushed on. The girl left alone with her baby sister and a grandma who became mentally ill had so much on her shoulders and still kept that youthful exuberance and positivity which requires so much strength and will. Along with that was the sad memory of her parents as shown when she played back the recording of her parents' voices for her little sister everynight for her to sleep, and the underlying curiosity and mystery of the circumstances surrounding their demise.
The fantasy element and colour in the movie also gave height to the experience of the film. It injected humour in its own way to help us look at the situation in an objective yet clear manner, standing as the third person only so very involved - intrigued.
All in all, wonderfully done and suitable for all ages. Not everyone would be as in love with it. It depends on how much you enjoy grounding yourself and how much you enjoy flight. Go watch and decide.