In the near future, Mother Nature has decided to fight back against all human encroachment.

The world has entered your standard apocalyptic scenario that you can find in many forms of fiction, but it's right in the middle of the apocalypse actually happening, rather than in the aftermath. Vines and foliage entangle cities, as large wooden mech-like creatures called Arethusa (named after one of the mythical Greek Hesperides that tended the gardens in a remote corner of the world) rampage through cities.

Souma Shin, a punk kid out of Japan, went to visit his brother Ken, but all hell breaks loose as Shin finds a group of small kids being used for military purposes. He tries to save them in the confusion created by an Arethusa invading the compound...