Plot: Amagi Mountain (also known as Saru-yama or Monkey Mountain due to its indigenous monkey population) is discovered to be the home of a 50 meter tall giant monkey, after the animal had ingested green leaf walnuts laced with Helipron G from a nearby research facility (Helipron G, a hormone nutrient developed during World War II for the purpose of strengthening soldiers in battle is found to trigger rapid growth in monkeys following excessive consumption).