This is a modern world, but one day gates or portals opened up, when nobody would kill the monsters in the gate it would soon have monsters flood the area. Soon random people would awaken and gain superhuman strength and powers, these people are called hunters and they go into gates and hunt the monsters. These hunters are separated into different classes the weakest is F and the highest is S+, people can awaken at any class and can climb classes but to climb classes is very difficult and rare.