The world is divided into two. One is a modern world where humans and animals live, developed through science and technology—"World A." The other is a mysterious world with an unknown ecosystem different from World A, inhabited by supernatural creatures that defy common sense—"World B." World A's science and technology have advanced, giving it a near-future setting. However, World B is not an otherworld that existed from the beginning. Long ago, a single researcher, driven by pure curiosity, conducted a massive experiment to unravel the "unknown realms of the world." That experiment split the world in two, giving birth to World B. World B cannot remain independent, and over time it erodes into World A. In areas where the erosion has progressed, special entities called "Cores" appear, and as long as a Core exists, that area is maintained as World B. If a Core is destroyed, the eroded area returns to the original World A. But at the center of World B, in the laboratory where the experiment was once conducted, there exists the "Primordial Core," the origin of all Cores. To completely stop World B's erosion, this Primordial Core must be destroyed. World B has two unique dangers. One is a phenomenon in which the bodies of humans who perished in World B begin to move without will or personality and attack the living. When warriors or adventurers perish, only their physical abilities and skills from life may remain, and sometimes close companions must fight against those bodies. The other is "supernatural creatures" that do not exist in World A. From giant beasts to creatures with unknown ecologies, their types are diverse, and not all of them are hostile to humans. To deal with the erosion into World B, the "Special Organization OXIS" was established in World A. OXIS is a specialized organization responsible for sealing off eroded areas, searching for and destroying Cores, dealing with supernatural creatures, and researching World B. OXIS's base is located on the World A side near World B, and around it, adventurers, researchers, engineers, and others have gathered, developing a massive city centered on the conquest of World B. Of course, ordinary citizens live there as well. Among those who venture into World B, people belonging to OXIS are called "OXISners," and those who do not belong are called "Adventurers." Adventurers have more freedom, but they bear full responsibility for everything.

World A of scientific civilization, and World B with its unknown ecosystem. At the boundary between them, people continue to challenge World B as "Adventurers" and "OXISners" to protect their daily lives.

As you is going about their daily life, an erosion alarm sounds.