Year 2036, Japan is under the adult party power and make children's life miserable. But for "rebelious-edgy-teenager-with-funny-scar" Kudo Eiji, It doesn't matter because he has his childhood friend Akari to cheer his everyday life. For Eiji, Akari is his emontional mainstay. Someone that can make him calm.
But then, one day adult party decide took Akari and transfer her to another school. It emotionaly stress Eiji out. Little did Eiji know, this awaken some power inside him

From Dan of Population GO:
An island was recently discovered off the coast of Japan. This island was mysteriously full of young men and women who lived as though they were in medieval Japan. The Japanese government immediately went to work enrolling them into modern Japanese schools. But not all is as it seems with these islanders. Could they all be descendents of the yokai expelled by the fabled Momotarou? Nahh, couldn't be.

From Mangahelper :

Humanity is on the brink of extinction as a flower known as “Viscar” infects humans and animals, turning them into mindless hosts that feed on others. If you get bitten by the infected, or inhale the pollen that emits from their bodies, the flower will plant itself into your brain. Humanity is fighting a losing battle against this world-wide pandemic, and most people have given up hope.

Volt was once part of the army and had been assigned to be the guard of Dr. Kisling, the scientist leading the development of a vaccine to fight against the Viscar. From an unfortunate accident, one of the contained Viscar pollen escaped, infecting Dr. Kisling. Dr. Kisling’s final wish to Volt was to find his daughter Lilith, and, with her help, continue the development of the vaccine. Lilith and Volt now venture into the post-apocalyptic world to bring back hope to humanity.