Satsuki Kakeru lost his sister by suicide and has been living a dull life ever since then. One day, he is transported into a bizarre world "Red Night" along with his friend Minase Yuka. In order to survive and escape from Red Night, he fights against Dark Spirits (Ralva) and Black Nights.
The story centers on Kakeru Satsuki, a seemingly ordinary high school student who suffers from amnesia and has lost all memories of his past. He lives a normal life until a supernatural event plunges him and his friends into a dangerous alternate dimension known as the Red Night. In the Red Night, ordinary people encounter terrifying monsters and hostile forces that seem intent on killing them. Those who enter the Red Night must fight to survive, but the city itself behaves strangely, as if governed by its own malevolent will.