In 2029, a massive ground subsidence occurred in the Mediterranean Sea. Chemical elements including oxygen, carbon, gold, molybdenum, and cobalt vanished suddenly from the Earth's crust. Over the course of sixty years, the human population declined by 90%. Researchers discovered that the missing elements had been drained into a planet called "Nega Earth," situated in another dimension. To save the Earth, a special unit known as the "Element Hunters" is formed. All members are under 13 years old, as young and flexible brains are required to access "Nega Earth."