In the year 2000, a Chinese nuclear experiment went wrong mid-transit, resulting in Japan falling into ruin. Those who survived were housed in Japanese based American military facilities to escape from the deadly radiation and learned to live following regimented lives of labor and learning.
To relieve the boredom, a recreational drug called "Zero" was created and sold illegally. Taking the drug induced a dream filled sleep where people saw their lives as they were in the year 2000. And in the cases of those who hadn't been born before 2000? The most they could surmise is that they saw who they were in their past lives.
But it seems something is brewing below the surface, and those past lives may just hold the answer.