New Orleans, Louisiana, 1893. A boy named Rizing is exploring the surroundings of his hometown together with a couple of friends. They discover a mysterious old house, go inside, and discover what seems to be a mummy in a coffin. They don't realize that at this moment they unleashed a powerful demon into the world, and that this demon has already possessed the soul of one of the boys...

The Apocalypse IV is the first Tengai Makyō game that is set outside of the Japan-like country Jipang. Much like Jipang of earlier games, United States of the late 19th century is not in a historically correct environment in this game, but an alternate world populated by demons, Biblical figures, and deliberate clichés (cardboard Native Americans, pop culture references, etc).

The gameplay largely follows the general Japanese RPG template and Tengai Makyō gameplay style in particular. As in the previous games, magic spells have to be sought out and acquired rather than learned or bought. A notable change is the absence of random enemies - all the enemies are visibly wandering on the world map or in the dungeons. Other gameplay additions include special attacks that can also performed by several members of the party as a combo, and obtaining money by selling trophies received in battles.