Flower Thieves is set thousands of years after a war between humans and demons devastated much of life on the planet, leaving the world as a scorched wasteland. In this dystopian future, the manga follows a large group of humans nearing extinction, residing in the overcrowded city of Upper Hose, where flowers and other flora are scarce and precious. When a plant-eating monster called a "Flower-Thief" attacks a mysterious girl named Jechika, a young boy, Maxi, is forced to use a forbidden ARM weapon to save her. He is subsequently expelled from the city for employing illegal technology. Accompanied by Jechika and a florist named Gi, Maxi ventures into the wilderness on a quest to restore the balance of nature across the world and render the earth habitable once more.