It's about four guys who are summoned to another world to become the heroes of the sword, spear, bow and shield. They need to gain experience and develop their magical weapons to fight waves of enemies that will appear and become progressively stronger.
But the Shield Hero is not respected there because he can't use weapons. And the heroes can't level up together because of some magical interference of their weapons. The Shield Hero can't do it on his own and most of the other adventures don't want to be with him, so he has a much harder way ahead of him than the other heroes.
There are a lot of unfair things happening in this world, like slavery, bigotry against demi-humans and no sign of a proper judicial system. It can get frustrating in the beginning, but it pays off later.
Edit: actually, there is no judicial system at all, unless you consider the ruler saying what they want.