Robotics student Ryuichi Asakura is studying in Germany, but finds himself socially isolated.
He is befriended by two other students, Maria and Rei. He immediately falls in love with Maria, however she and Rei also have feelings for each other.
Ryuichi and Rei also bond over the experience of being Japanese in Nazi Germany, and promise each other to let Maria decide.
A year later, Rei enlists in the German army, while his friend Asakura goes to work on the ЯR Project that spawned 'Proto', with the project entering Phase 2.
Meanwhile, the cyborg super soldier project is in jeopardy because of Proto’s disappearance and Minus One's death. Ryuichi realizes that the missing cyborg bears an uncanny resemblance to Rei.
Out of jealousy and seeing an easy way to remove his romantic rival, Ryuichi reveals this to the general in charge of the project. Rei is abducted and turned into a cyborg, his memories are wiped and replaced with Proto's data.
Rei, now became Humanoid Assault Weapon Phase Zwei, known as "ZERO". Zero is now the Grim Reaper of Nibelung, and leads a new battalion, this time comprised of cyborgs like himself called "The Ghost Unit".
Zero began to remember his most precious memories when he was human and slowly retains his humanity. This would lead to an eventual revolt against his creators.
