"If only we hadn't rescued him..."
There is a high-speed experimental ship, "Vasilisk." The protagonist, Kouzuki Kyousuke, and his friends happened to be on its trial voyage. It was summer at that time. Because the trial voyage's supervisor was also the mother of the protagonist's childhood friend, Kyousuke and his friends were spending their time as if they were on a cruise.
One day, the ship came across an ocean investigation ship adrift on the high seas. There was only one person on its board: Ishida Youichi, who claimed to be an oceanographer. He was a well-mannered young man, so there were no particular issues with him. However, as soon as they took him abroad, the ship's navigation instruments began to fail one by one.
And when the sun was hidden under the veil of darkness, an incident occurred—there was a sudden blackout. Once the electricity was restored, a cruel scene appeared before the eyes of Kyousuke and his friends. There was a woman tied to a chandelier. She was in terrible shape, as if she had been tormented mercilessly.
That woman was the supervisor, Orihara Shino...
[Translated from the official site]