Attending the serene Enishida Academy, an isolated boarding school on a lush, walled-off island, Fujisako Aiko is a nice but solitary girl. While exploring the island's forests, she catches sight of two foreign-looking girls about her age in a clearing. One has a cold, pretty demeanor while the other is cheerful and happy. Despite their differences, Aiko discovers they are sisters from a royal family of another kingdom. Welcomed into their lavish lifestyle and large household of siblings, she forms a bond with her new companions. However, Aiko senses a subtle wrongness and finds herself compelled towards the coldest sister with obsidian eyes.
