The Allegory of the Cave is one of the more famous parables by Plato, where he imagines a group of people chained in a cave, knowing only the shadows on the wall in front of them. He says they would presume that the shadows were the real world, having known nothing else. He then asks us to imagine a prisoner who broke free. He would first have a hard time looking at the light of the fire, because his eyes wouldn't be used to it. He would think the fire were less clear than the shadows. When he walked outside into the sun, he would be even more dazzled by the brightness, but with time would come to understand the "true world". Plato then says that he would be obligated to go back into the cave and teach the prisoners about the true world, even though they would mock him and be reluctant.

You can watch a great animated version narrated by Orson Welles here.

Movies like The Matrix are said by some to have similar themes, and much, much more Kung Fu. Plato probably could have learned a thing or two on the Kung Fu front from The Wachowskis, when you get down to it.