Plato's main message in the Cave of Plato is that when we seek for greater level of knowledge or truth, we will never see the same way as we did before.
This Cave could be a representation of the inner side of society because there is more to the world than just the cave.
The main message in this allegory could be of the human mind. This is because the story is around the individual that would like to know more.
The main message in this allegory could also be of how society works. This is because the prisoners are simple people who only accept what they see or what they have been taught and know. But the person who wants to know more of the truth went out of the cave and became to learn more and when he returns to the cave, the others deny what he has to say and laugh at him/her for saying these things.