In 2017, the humans made contact with alien life forms. In response to them, the aliens attacked the humans. It was the Earth Defense Force that began their counterattack, although the aliens eventually pinned the humans into a corner. In a desperate attempt, the humans made a strike at the mothership and without it, the aliens left.
The humans have begun rebuilding their civilization, but they are doing it with the alien technology, which ironically helped out a lot. However, that peace didn't last too long, as on June 28, 2025, a state of emergency broke out. It turns out that many of the aliens had buried themselves underground and began breeding. It is now up to the Earth Defense Force to vanquish these aliens once again.