This manga series was adapted into an excellent anime in 2004--that's where I first learned of it.
The anime was never licensed here in America, but the manga was. The manga is hauntingly good.
It's a complex story spanning something like 4000 manga pages, with lots of different themes and motifs. At its base, it's the quest of a doctor to right the wrong his own righteousness created. It's full of mystery, the shadows of tyrannical dead governments, life on the run, and questions of morality and philosophy. There are segments of high tension and dread, and equally well-handled scenes of explosive action.