Six years ago, Tsukihiko was caught in an assassination attempt on a little girl, Sami, when he attempted to be her shield. This incident nearly killed him and the girl together, but the little girl was the daughter of an extremely powerful family. With its vast amount of resources, the family was able to keep both Tsukihiko and Sami alive by recreating both of their bodies with cyborg technology. This has forced Sami to permanently rely on wired connection to her life sustaining computer. On the other hand, Tsukihiko has become a cyborg "Knight," sworn to protect Sami from harms way.

(Source: MangaHelpers)