Set at the end of the 20th Century. Because the Japanese Empire, an island nation in the far east, was conquered by Caesar, a resurrected hero from Ancient Rome, it is now controlled by the neighboring Eastern Roman Empire! The power that made Caesar the most powerful conqueror was called Legion. That is, a mysterious army composed of gathering all the great powers; a giant winged army. The Japanese Empire's imperial princess, Fujinomiya Shiori, conceals her ambition to become the supreme ruler and begins to act. The person she chooses as her trusted commander is, at first glance, the mediocre high school student Tachibana Masatsugu. However, he too is someone reincarnated from the past; a hero whose greatness rivals even Caesar's: Legatus Legionis!!