Technology, science, and industry—this is Tokyo in 1969, and there has never been a finer moment to be alive! However, a darkness lurks around every corner: "Invaders" have infiltrated the community, and no one knows who or what they are. Only members of the top-secret AEGIS agency are aware of their existence. Their goal is to battle the enemy covertly, but only those with the capacity to open "Gates" to another realm can actually defeat them. The "Gate Keepers," a specialist task force under AEGIS, are the only ones who can save mankind from the terrible Invaders afflicting the globe. They are made up of amazing individuals with a range of Gate-related talents. Shun Ukiya is a typical high school student who lives with his widowed mother and two younger siblings.