Yoshirō and Mumei in sealed Japan
After an unnamed catastrophe, Japan has sealed itself off from the world. Its elderly have become effectively deathless and still work past 100, while its children are born frail. Over a century old, the writer Yoshirō raises his great-grandson Mumei, who cannot chew bread, drinks only certain juices, and walks with brittle bones, yet is cheerful, and the boy is eventually chosen for a secret mission abroad as an 'emissary' whose body might help foreign researchers.
