The story is set in 1969, a period of time in which Japan is experiencing intense economic and social development after the end of World War II in 1945.

Unbeknownst to humans, alien/interdimensional beings have emerged with plans to take over the world by sending numerous agents to wreak havoc in cities, turning people into robotic minions.

In an attempt to confront the Invaders and defend the planet, an elite but highly secret organization of peace-keepers called the Alien Exterminating Global Intercept System (more commonly known as A.E.G.I.S.

lies beneath Tategami High School, a facade created by the organization to mask the active recruitment of new Gate Keepers.

A three-volume manga series, created by Keiji Goto and following the plot of the video game, was published by Kadokawa Shoten.