Touya Misaki is a female student at the prestigious private Aigaoka Academy. Her parents died when she was young, and she lived in an orphanage for some time, but was adopted by a childless couple. Raised by kind-hearted parents, she spends her days with her childhood friends, the Uryuu sisters, Kazuho and Futaba. However, there is one and only one concern on her mind: her sister, who has been separated from her.
Misaki has a twin sister named Hyouka, but she was adopted by another family before her, and they have not met ever since. She is sure that her sister is living as happily as she is. She believed so, and was planning to look for Hyouka when she eventually become old enough to take care of her adoptive parents, and yet...
The day of their reunion came suddenly. One day, a transfer student arrived at the school. It turned out that the new student is her sister Hyouka, whom she had been separated from. Misaki was happy about the unexpected reunion. In contrast to her, Hyouka was perplexed. Misaki was bothered by her reaction and investigated the family that took her sister in.
She found out that Hyouka's foster father was the president of a large company called "Kijima System." It is a large company with a wide range of operations, but there are also black rumors flying around it, which makes Misaki somewhat uneasy.
And Misaki's fears end up being right on target, as she discovers the awful truth...
[Translated from the offical site]