Dr. Koutaro Azuma created Android BK-1 to protect the Earth. In following its primary directive, to protect the environment, BK-1 determined that humanity was a threat to Earth. Renaming itself Black King Boss, BK-1 created an army, the Andro Corps, to destroy all human life.
As humanity falters, Tetsuya, son of Dr. Azuma, initiates his father's last experiment to create the ultimate android. He merges with the machine, turning into Casshan — the symbiosis of human and machine. Casshan has one single goal, to eradicate the Black King Boss. On his travels, Casshan is accompanied by an advanced transforming robotic dog named Friender, a faithful fighting assistant, but also an affectionate reminder of Tetsuya's dog, Lucky.
While trying to reach BK-1 Casshan encounters human resistance member Luna Kouzuki, who possesses a weapon capable of defeating the Andro Cops, trying to get it back to the human army. She has also been searching for Tetsuya while fighting the robot army. When they finally meet, Luna is confronted with the fact that Tetsuya is dead while Casshan confesses to her his fear of becoming another BK-1.
With humanity's future on the line and Casshan's humanity slipping away, will he be able to defeat BK-1 or will he join BK-1's army in the end?