Hunter of Monster Special Force, was originally a group of scientists from Japan lead by Yamato Takahashi, tried to research ways of protecting Japan from the increasing attacks of atomic super-monsters. In doing so they found the lab of Robert Hellsgaard, while there they opened the portal to Limbo and freed him from his prison.
With Hellsgaard's technical support, they went on to kill all the super-monsters in Japan. After that they killed vampires, werewolves, yetis and poltrageists, but eventually they started to kill mutants and even disfigured people.
After exterminating the monsters on Monster Isle, they discover and proceed to attack Monster Metropolis which contained the Legion of Monsters and the newly revived Punisher (who was rebuilt as Frankencastle).