Kumon Katsuki feels as if he was born without a heart. Ever since he was young, he's had a hard time reading the mood or empathizing with others; having to resort to using his brain instead of his emotions in order to get by in social situations. Because of this inability to read the atmosphere, the only people he has been able to maintain meaningful relationships with are his Landlady, Hanawa Momiji, and his two friends, Mineyuki Ryou and Makimoto Misae.

One day, Katsuki's younger sister, Megumi, decides to visit him from abroad. What should be a joyous reunion, however, coincides with a series of bizarre incidents occurring around Katsuki.

Among them are a series of strange dreams where he meets a young woman with an umbrella; only to have an encounter with her in the waking world. On top of that, a series of violent murders has begun sweeping the town Katsuki lives in—the victims looking as though they've been mauled by some large animal. However, the strangest of them all is an encounter with an enigmatic woman who claims that Katsuki is something called "The Gate".

Will Katsuki survive the series of event he's now found himself the center of, or will he meet his end?