The long and short of it is the film presents a future about 200 years ahead after environmental ruin (from war, pollution, etc) have forced survivors into a domed city. The inhabitants, to the best of their knowledge, have always lived there. They are dimly aware of a world outside the city, but have no way of comprehending it or conceptualizing it. In this city, the majority of tasks are automated and humans live mostly for pleasure.
Because of the need to keep the population stable, adult life ends at 29 years 364 days. On this day, called Last Day, the would be 30 year-old citizens step into the Carrousel hoping to be "renewed." It's never actually explained what renewal is, if it is a reincarnation or something greater, but the majority of the population believes it to be true unswervingly. As expected, no one who enters the Carrousel ever makes it out alive. Therein is the population control set up by the city founders long ago.
At the time the story begins, the residents are largely looking at the 6th and 7th generations since the city's founding. One must wonder if any of the original city's founders actually went through Carrousel themselves in a grand sacrifice to show the 1st generation what the belief system was and what was required. Difficult to say.
I'm not going to go into the majority of the film, but the ending I'd like to touch upon. Sufficed to say, at some point our stalwart protagonists decide that Carrousel is a lie, and the residents of the city need to be freed so they can live life beyond 30 and see what lies beyond the dome. The how and whys of this are best left to the film, but in the end, the city's central computer suffers a catastrophic failure and the residents of the city are forced to flee to the outside world.
This is meant to be a good thing in the tale the film presents us. Freedom wins out, life can take its natural course instead of being rigidly controlled, and all that.
I can't help but wonder how long it will take for humanity to die out now that they have left the domed city. You have a population of say, 3-to-29 year old people who have never worked, never really been educated, and have no survival skills suddenly thrust into a world where humanity has been absent for two centuries.
They have little concept of bacteria or viruses, no advanced medicine, and no natural immunity to any of the allergens they will encounter. All of this was provided in the sterile environment they were raised in, and medical care was of an automated technological means none of them has the understanding to replicate. Nor, if they did, would they have the tools to replicate it.
None of them are skilled tailors, weavers, or hunters, and the clothing they will take with them from the city is flimsy and light.
None of them have had to chop wood to build a fire, and not a single one of them has any concept of how much wood will be necessary to survive a winter. None of them even knows how to survive a winter. They don't understand that by relocating south, their chances of survival would increase as the climate would be warmer, because they have no understanding of climatology or meteorology.
They have never had to hunt, farm, or gather fruit. They do not know which plants are toxic.
While the human animal is adaptable, the history of our species is thick with the blood of those who could not survive the lands they were sent to colonize. Will any of them survive to see the next year?
I remember when I first saw this film through much younger eyes that I was happy that the citizens had beaten the Life Clocks and could live on.
Watching it now through much older eyes, I fear for those citizens, and am cognizant of the horror that awaits them.