In 1945, Dr. Ryūichirō 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 put Metalder to sleep in the Silver Caucus base and left for the United States to work for NASA.
During the 42 years, Dr. Koga's former colleague Kunio Muraki became the evil God Neros, the ruler of the Neros Empire which is notorious as a merchant of death. God Neros sends out his massive armies to kill Dr. Kōga, who learned of their organization and returned to Japan to activate Metalder. Koga desperately tries to activate his android creation, whom he named Ryūsei Tsurugi, but the android is unable to understand what to do. In order to give Ryusei a "purpose", Koga runs out of the Silver Caucus base, allowing Neros' troops to kill him. Suddenly seeing his creator die, Ryūsei becomes shocked, and soon is beaten by Neros' troops. Ryūsei becomes fueled with rage, evoking the conversion to his true form, the android known as Metalder.
