In 1969 Tokyo, amid rapid strides in technology, science and industry, life feels at its zenith. Yet a quiet menace lingers from every corner: the "Invaders," whose true nature remains unknown.
Only AEGIS, a covert agency, knows of them. Their clandestine mission is to neutralise the threat, but only those who can summon "Gates"—portals into another realm—hold the power to defeat it. AEGIS"s elite squad, the "Gate Keepers," is made up of remarkable agents with diverse Gate talents; they alone can protect humanity from the relentless Invaders.
Shun Ukiya, a typical high‑schooler sharing a home with his single mother and younger sister, is suddenly confronted by Invaders on his way home. In a frantic bid to protect them, he discovers he can open a Gate and channel vast power. This revelation draws AEGIS"s attention, especially from Ruriko Ikusawa—Shun"s childhood companion and a member of the Gate Keepers.