In 2008, the appearance of the first monster "Gomes" heralded the beginning of the "Monster Frequent Period", and alien invasions also occurred from time to time. Human countries finally united to establish the "Earth Defense Force" EDF, a global armed force designed to defend against monsters and aliens. But as to why "monsters appear frequently" occurs, experts from various countries have different opinions and cannot agree.
In 2012, human beings successfully established diplomatic relations and launched a series of cooperation with the cosmic beings called "Deslei Nebula People". As a result, human science and technology ushered in a blowout.
In 2023, specifically in response to the high frequency of monsters and high urbanization rate in Japan, the EDF Japan branch established a special unit "Aersa" composed of elite personnel to deal with monsters in the nature of a special operations detachment, adopting this elite squad-style combat tactics and model.
In 2024, the mysterious ancient glory shines, and our story also begins...
