Taking place in a not-so-distant future, humanity's incessant expansion has provoked a retaliatory response from Mother Nature herself. The typical post-apocalyptic setting is replaced with an immediate and ongoing crisis. Buildings are overrun with vines and verdure while colossal, mechanized beasts known as Arethusa, named after one of the Hesperides from Greek mythology that tended to a distant, lush oasis, run riot through cities.
Follow the story of Souma Shin, a rebellious youngster hailing from Japan. The narrative unfolds with his arrival to visit his twin brother Ken, but he quickly becomes embroiled in a chaotic situation. In the midst of an attack by a rampaging Arethusa, Shin encounters a group of innocent children being forcibly drafted into military service. Determined to save them, he takes action.