an age-old conflict between those who inhabit the Crimson Realm (紅世 Guze), a parallel universe to the human world. The Crimson Realm is populated by Crimson Denizens (紅世の徒 Guze no Tomogara) who are able to manipulate the Power of Existence (存在の力 Sonzai no Chikara), a fundamental power within any biological entity and functions as "fuel" for one's existence. Denizens whose power stands out among their peers are known as Lords of the Crimson Realm (紅世の王 Guze no Ō), and Lords that are even more powerful are known as Gods (神 Kami). Denizens who do not care about the balance between the Crimson Realm and the human world collect Power of Existence from humans to use for their own purposes. It is the duty of Flame Hazes (フレイムヘイズ Fureimu Heizu), humans who have formed contracts with a Crimson Lord, to maintain the two worlds' balance by killing any Denizens disrupting it. Denizens and Flame Haze fight with the use of powerful magic spells called Powers of Unrestraint (自在法 Jizaihō), which are also known as Unrestricted Spells or Unrestricted Methods. One such spell is the Seal (封絶 Fūzetsu), which creates a space where the Crimson Realm and the human world intermingle and all sorts of causalities, including time, are stopped for ordinary living beings.

The story largely takes place in Misaki City in Japan and begins when high school student Yuji Sakai is thrown into the middle of this conflict and encounters a Flame Haze girl with flaming red eyes and hair dressed in all black wielding a katana. The girl informs him that he died some time ago, and that he is a Torch (トーチ Tōchi), a human with greatly diminished Power of Existence, and whose remaining Power of Existence will slowly run out. In addition, Yuji is a Mystes (ミステス Misutesu), a special kind of Torch who contains a Treasure Tool (宝具 Hōgu), a magical object created by Denizens that can provide special abilities or even Powers of Unrestraint; additionally, Mystes can move within Seals. Unfazed by his apparent death.