Another idea-

"The world isn't what we're shown. It's far more dangerous and forbidden than we know."

Kim Namjoon always knew something was wrong with him when he questioned things people never thought of. The curiosity was killing him until he found an ancient box, full of forbidden books.

They were all living in a lie, he realized.

With the help of 6 more boys who were curious about the world, he made a promised to pass the four walls of the prison and know the real world outside.