The alien invasion force called WARSTAR destroy the Heaven’s Tower, the bridge between the Earth and the Gosei World, home of the Gosei Angels. Five apprentice Gosei Angels who happened to be on the Earth transform into GOSEIGER and fight to defend the Earth.

An accident during tests of an anti-plasma artificial magnetic shield at Japan's Ground Self Defense Force East Fuji practice range sends the 3rd Special Experimental Company, under Colonel Matoba on a time-slip 460 years into the past, into 'the Age of civil War'. At the same time an imaginary-number anomaly thought to be caused by interference from the past begins eroding the present.

When a crew of bank robbers believes that an alluring young woman has blown their cover, they kidnap the girl. Back at their hideout they develop sinister plans to rape and then kill her. When one of the crooks mysteriously turns up dead however, they begin to realize that their intended victim may be a powerful, vindictive, supernatural force.

Nathan Algren is an American hired to instruct the Japanese army in the ways of modern warfare, which finds him learning to respect the samurai and the honorable principles that rule them. Pressed to destroy the samurai's way of life in the name of modernization and open trade, Algren decides to become an ultimate warrior himself and to fight for their right to exist.