World Trigger [TV]

In Mikado City, a “gate” to a different world is opened suddenly one day. Monsters called “Neighbors” start appearing from it, everyone is afraid of them because Earth’s weapons don’t work against them, but a mysterious group starts fighting off the Neighbors. They, the “Border” defense agency, create a defense system against the Neighbors. Since then, regardless of the Neighbors still emerging from the gates, people of Mikado City were living normally today. Four and a half years after the gate is opened for the first time, Y?ma Kuga, a humanoid Neighbor, comes to Mikado City, where he meets Osamu Mikumo, a Border agent.

(source: http://en.wikipedia.org/wiki/World_Trigger)