From MangaHelpers:

The introduction of western culture into Japan brought extreme changes to their society. From shifts in political power to the people quickly accepting Christianity, the country saw drastic transformations. However, there were just as much people accepting the change as there were people opposed to it. The pirate Iori has been rebelling against the westerners and those allied to them. During one of the raids, Iori finds two foreign prisoners and saves them. They are secretive about their identity but Iori soon becomes invested in their mission. [vyc]