Ten years ago, an alien portal known as the Space suddenly appeared. Within this extradimensional realm where aliens are said to dwell, a legendary alien relic called the Garakuta is rumored to exist, capable of granting any wish. High school student Ao Akashiro ventures into this forbidden territory, driven by the need to find her missing sister. She is accompanied and aided by a mysterious black cat named Kuro, with whom she shares a telepathic bond.

The story takes place in a version of Earth that has been transformed by strange radiation from deep space. These changes have created areas known as the Space, where the normal laws of physics do not apply and almost anything is possible. While Ao searches for the artifact that can grant her wish, she discovers she is not alone. Other beings and rival individuals are also hunting for the Garakuta, leading to violent conflicts within these dangerous, reality-warping zones. The narrative blends science fiction and horror elements, as the ordinary school setting gives way to an unpredictable struggle against parasitic invaders who observe, learn, and adapt to the students around them. As the situation escalates, classrooms and corridors transform into battlegrounds, forcing the students to form fragile alliances while trying to survive the ongoing incursion.