If you don't want to read the whole story, basically |there's a utopian society where everyone is happy, intelligent and cultured. The catch is that there is a human child who is kept in perpetual filth, darkness, and misery, and this is the only way to maintain the utopia. If the suffering of the child is ended, the society will collapse and everyone else will suffer greatly. The story ends by reflecting on those who decide to leave the society rather than to continue benefiting from the suffering of the child.
Really worth a read