In Mikado City (三門市 Mikado-shi?) (280 000 inhabitants), a "gate" to a different world is opened suddenly one day. Monsters called "Neighbors" (近界民ネイバー Neibā, lit. "Near-world people"?) start appearing from it, everyone after the gate is opened for the first time, Yūma Kuga (空閑 遊真 Kuga Yūma?), a humanoid Neighbor, comes to Mikado City, where he meets Osamu Mikumo (三雲 修 Mikumo Osamu?), a Border agent.