In the year 192 of the United Earth Calendar, mysterious creatures from outer space invaded the Earth. They wreaked havoc and slaughtered humans. The Earth turned into a sea of fire, and human civilization was in danger. One day, the outer space development base found and rescued a man who fell from the sky. From the man, they learned that the alien invader was named "Radam", but they knew nothing about the alien's motives and the man's origins. Soon after, people discovered that this man actually had a strong power to fight against the invaders, and the space knight he transformed into was very similar to the brutal warriors of "Radam"! The mysterious man codenamed D-Boy, who is he? The savior? Or the terminator?