Five years ago, portals appeared and brought monsters into the world. At the same time, a lot of humans gained superpower to fight those monsters. However, the college student, Garren Gates, only got the power of a Gatekeeper--a non-fighter with the ability to stabilize and close portals. Even though he always feels inferior, Garren has succeeded in becoming the Gatekeeper of the Hero's Guild! And he will prove his worth in his first S-rank mission!