In the story, Gaimetal, a metal ore with high amounts of pure energy, was discovered 50,000 meters (160,000 feet) underground in 2047, and mining began all around the world. However, Gaist, metallic lifeforms whose bodies are entirely encrusted with Gaimetal, emerged in 2055 and began attacking people. In 2064, Rekka, Hayato, Kurama, and Siren are among those who suit up in Gaist Gear and battle over 100 varieties of Gaist creatures.

(Source: Anime News Network)