From the back cover of game:
In a tower rising high above a desolate city, a mad-genius scientist pursues his experiments. His mission: to create the ultimate killing machine and purge the world! The means of achieving his wild ambition will soon be set in motion. All that remains is the last piece of equipment for Tetsujin... and YOU have been chosen. The assimilation process is underway. The scientist awaits you. Head for the top floor of the tower!