30 years ago, in the year 2020 the world was shaken by an unprecedented event. Thousands of cities saw black lights spiraling on the air. Brave people who tried to touch the lights disappeared in front of many eyes. The governments of countries affected by this phenomenon deployed military personnel to restrict people from approaching these spiraling black lights. For a few months, nothing happened. The world's best scientists cannot explain the strange phenomenon. That is until a person suddenly came out of the black light in Kyoto, Japan. That person was Keisuke Kagawa, now known as the First Defender.

After the reemergence of the First Defender, other people who disappeared due to the black lights started to reappear from those black lights in the following months. Although, not everyone who disappeared managed to came back. From the people who came back, the world learns the truth about those black lights. They are portals to another dimension known as Realm of Defenders.

The Realm of Defenders is a realm full of horrifying and wonderful things at the same time. In there it's possible to learn how to wield magic, evolve a person's physique, and obtain various items with wondrous effects previously thought impossible for mankind. But, to obtain all of those a person needs to struggle in the face of death. Just to obtain a return stone that enables someone to return to Earth requires them to face death multiple times. Those who were unable to return were those who died trying or those who gave up.

To this day, 30 years later, nobody knows how the Realm came about. All kinds of theories were published: government conspiracy, alien technology, time travel, divine intervention. But nobody can confirm which one is true. Nevertheless, everybody in the world accepted its existence, because the Realm of Defenders enables Earth to enter a rapid growth in terms of technological advancement. All new kinds of technologically advanced and magical items could be found inside the Realm.

Unimaginable wealth and power can be obtained by being a Defender. Defender, it's a term used to describe people who explores the Realm. It's a term assigned by the Realm itself, but nobody knows why it's called Defender instead of other terms like Explorer or Adventurer. What are they supposed to defend? There is no answer to this question until now.