In the summer of 2016, an unknown alien organism invades Earth and annihilates 20% of the human race in a span of one month . However, survivors of this virus were either transformed into a hideous creatures called "Worms" or into super-humans called the "Heteros". The heteros form a protection agency called the "ISO" to fight the worms. Haru, a young hetero but also a mischievous trouble maker, wants to join this agency in the fight. But it's no easy road.