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Summary
In the bustling city of Mikado, a portal to another world opens, allowing hostile creatures known as Neighbors to invade Earth. Traditional weapons prove ineffective against these aliens, prompting the formation of the Border Defense Agency—a group armed with special weapons called "Triggers" designed to combat the Neighbors. Even after several years, the Neighbors continue to pose a threat, and Border agents remain vigilant to protect the city.
Osamu Mikumo, a Border agent-in-training, is not permitted to use his Trigger outside of the Border headquarters. However, when a mysterious new student, Yuma Kuga, is dragged into a restricted area by bullies and subsequently attacked by Neighbors, Osamu steps in to help. To his surprise, Yuma quickly eliminates the aliens, revealing himself to be a humanoid Neighbor. Together, Osamu and Yuma form an unlikely partnership, navigating the complexities of protecting the city while confronting Yuma's enigmatic past.
