The earth of the distant future is covered in thick clouds, and the sun no longer shines. Plants wither, and oxygen is thin. To fight extinction, humankind has developed a technology that turns humans into plants, providing a small amount of oxygen. Is this process sustainable? Is it ethical? Toshiro Kamiya must consider these questions as he's faced with a difficult choice--save his family or save himself.

Kamiya is trying to make the most of the time he has left before completing his spiriflor transformation. But when his childhood friend Hourai becomes the latest victim of a mysterious spiriflor that somehow remains capable of humanlike movement and functioning, he finds himself drawn into the hunt for this deadly creature.