Yoshiro thinks he might never die. A hundred years old and counting, he is one of Japan's many 'old-elderly'; men and women who remember a time before the air and the sea were poisoned, before terrible catastrophe promted Japan to shut itself off from the rest of the world. He may live for decades yet, but he knows his beloved great-grandson - born frail and prone to sickness - might not survive to adulthood. Day after day, it takes all of Yoshiro's sagacity to keep Mumei alive. As hopes for Japan's youngest generation fade, a secretive organisation embarks on an audacious plan to find a cure - might Yoshiro's great-grandson be the key to saving the last children of Tokyo?.