The world has become a slaughtering ground for the Crimson Denizens, mysterious beings from a parallel universe who thrive on the life energy of humans. These merciless murderers only leave behind scant remainders of souls called "Torches," which are mere residues that will eventually be destroyed, along with the very fact of the victims' existence from the minds of the living. In an ambitious endeavor to put an end to this invisible, hungry massacre, warriors called Flame Hazes relentlessly fight these monsters.

One fateful day, Yuuji Sakai ceases to be a regular high schooler—he becomes trapped in a crevice of time and is suddenly attacked by a Denizen. Coming to his rescue just in the nick of time is a nameless hunter who seems no different from an ordinary young girl except for her blazing eyes and burning crimson hair. However, before Yuuji can learn anything more about his situation, he discovers that he has already been reduced to a Torch—merely a scrap of memory waiting to be extinguished.

In this world, there exist demon swords. Those who have the sword will have their will taken over, and they will lust for human blood and keep on killing indiscriminately. The sealer is who searches for those blades that haunt people and seal them with his scabbard.

Shi Yi was once feared by men as a bloodthirsty demoness, killing hundreds. No one could stand in her way. But scared of the woman she'd become, Shi Yi's treasured blood slave Yi Ling stabbed her and imprisoned her in ice. Now 200 years later, Shi Yi is free from her prison but with no knowledge of who she is. Still plagued by blood rage when she uses her powers, she finds her way into the care of the seemingly familiar Hua Yu Tang, who promises to heal her from this mysterious ailment.