"Gates" are portals that appear around the world. From these gates pass various demons and monsters that terrorize humans. No one knows what is past these gates, only that what comes from them are threats to humanity. It is up to the "gate hunters", people with supernatural abilities to fight off those that emerge from the gates and protect the populace. They must travel from one place to another, recruiting members, and destroying these gates. Step by step, they try and figure out the mystery behind these gates and the beings that come out of them.