Zennou No Noa - The World Of Lost Memories summary:

In the year 2038, a white light descended on the center of Tokyo and stole the memories of over one million people. The cause of this horrible incident was not determined, and no cure was to be found. The government formed a separate city beneath Tokyo to house and care for the victims of the incident and branded them as "KIDz." Regardless of their age, they truly are like children, and they must be retaught everything they once knew. Noa Ikurumi, a boy genius who lives in this isolated society, is just taking one day at a time and dreaming of finally being allowed to go back out into the world. But Noa's life will once again change drastically on the day that he is taken hostage by a terrorist with some surprising information about his identity...