During a school cruise trip, a group of friends are thrown back to the year 1636. According to history, in 1637 a revolt known as the Shimabara Revolt broke. It was a rebellion by Christian rebels and led by a charismatic youth known as Amakusa Shirou, which ended in the defeat of the rebels and resulted in the loss of 3,000 lives. Hayumi Natsuki, our heroine, is mistaken as Amakusa Shirou by the villagers she met, never mind that the real Amakusa Shirou is male. Apparently the real Amakusa Shirou died the year previously, before gaining recognition or popularity as a leader among the people. Before long, she became famous as some-sort of heaven-sent angel and people started calling her "Shirou", believing she is the saviour prophesied 25 years before.

(Source: Anime News Network)