From Evil Flowers:

During their senior trip on the cruise ship Victoria, a group of friends is suddenly pulled back in time and separated. The main character, Hayumi Natsuki, arrives in the year 1636. In Japanese history, Christian rebels, led by a young man known as Amakusa Shirou, revolted in 1637, which resulted in thousands of casualties.

However, when Natsuki arrives in the past, she and her friend Eri discover that Shirou Tokisada is already dead. Meanwhile, Natsuki, according to the villagers and Shirou’s parents, looks exactly like their late son (disregarding the fact that Natsuki is female).

Natsuki, the kendo champion of her school and heir to her family’s Hakuhouryuu kendo style, simply cannot stand by and watch innocent people be persecuted simply for being Christian. Soon she even becomes known as an angel, sent from God, whose arrival was prophesied 25 years earlier. But what will she do when old friends become new enemies?