Shana is a Flame Haze, an agent bound to a Crimson Lord and charged with hunting the Denizens that devour human existence. Her hair and eyes turn red when she draws on that power, which is where the title of the original series comes from. The game places her in the Shadow World, a realm reached from the ordinary one, where the fighting takes place without the wider population noticing anything has happened.
The player raises Shana rather than commanding a large party. Equipment taken from battles is enhanced to raise her statistics, artifacts are collected and equipped for passive effects, and Unrestricted Spells are learned to give her active abilities. Any piece of equipment can also be worn purely for its appearance, so the way Shana looks is decided separately from the build being used, and the outfits other players have assembled can be viewed for reference.
More than twenty characters from the anime appear over the course of the story, including Yuji Sakai and Hecate. Several of them fought against Shana in the source material and are recruited as allies here, assigned alongside her to contribute their own skills during a stage. The scenario is written for the game and avoids the central events of the anime. Materials needed for growth accumulate while the game is closed, so progress continues between sessions.