In 1945, Dr. Ryuichiro Koga designed Metalder as a top-secret weapon for the Japanese Imperial Army for use in the Pacific War, modeling it after his late son, Second Sub-lieutenant Tatsuo Koga. However, the pacifistic Dr. Koga puts him to sleep in the Silver Carcass base and left for America to work for NASA. During the 42 years, Dr. Koga's former colleague Major Muraki became the evil God Neros and sends out his massive armies to kill Dr. Koga, who learned of their organization and returned to Japan to activate Metalder. Koga desperately tries to activate his android creation, who he named Ryusei Tsurugi, but the android is unable to understand what to do. In order to give Ryusei a "purpose", Koga runs out of the Silver Carcass base, allowing Neros' troops to kill him. Suddenly seeing his creator die, Ryusei becomes shocked and fueled with rage, evoking the conversion to his 'true' form, the android known as Metalder.