Lucifer, an Archangel and former head of the Seven Heavenly Virtues, is banished from Heaven after revolting against the Lord's will. While plummeting from the skies, she is halted halfway between Heaven and Hell after crashing through the roof of a high school church. Though she is witnessed by Maria Totsuka, a soft-spoken student at the academy, Lucifer swiftly continues her descent into the depths of Hell. Soon after her arrival, Lucifer is found by aspiring Demon Lord and fangirl Leviathan. The two decide to overthrow the Seven Sins, the authorities of Hell under the leadership of Belial. But with their combined powers, the Seven Sins are able to repel Lucifer and contain her divine powers by placing a Garb of Punishment over her body, transforming Lucifer into a Demon Lord. Longing for revenge and accompanied by Leviathan, Lucifer makes her way back to Earth, where she forces Maria to become her immortal slave. Together with her new accomplices, Lucifer sets out on a mission to subdue the Seven Sins so she may be free of the curse brought upon by her Garb.

The Gates of Hell reopen, throwing the world into chaos and setting demons free to terrorize the land once again. These troubles seem irrelevant to quirky villager Touka Scott—until a demon attacks Yuna Eunice, his childhood friend. Luckily, the legendary hero Sion Blandan arrives in the nick of time to strike down the vile creature. Unfortunately, Sion soon dies after accidentally falling into a trap Touka set to defend his radish farm. Anri Haysworth, Sion's necromancer companion, resurrects the hero by transferring Touka's soul into Sion's corpse. Now possessing the body of the legendary hero, Touka is tasked with resealing the Gates of Hell and defeating the demons that threaten peace. Together with Yuna and Anri, Touka copes with both his new reality and Sion's heroic legacy as he embarks to save the world.

As dungeon-like treasure vaults started appearing throughout the land, more and more people began signing up as treasure hunters—risking their lives to obtain the vast riches and ancient relics inside. In the imperial capital of Zebrudia, the top hunter party is the Grieving Souls—a group of six childhood friends led by Krai Andrey, who is thought to be the strongest of them due to having a reputed wide array of skills. However, Krai actually has no special talent. His accomplishments are actually the result of his friends being overpowered. Krai has repeatedly attempted to leave the party out of fear his lack of aptitudes would one day cause his companions' downfall. Nevertheless, everyone around Krai thinks differently, making him feel obligated to remain the group's leader.

During a visit to the National Library, Miaka Yuuki and Yui Hongo stumble upon a strange old book that casts a red light, sucking them inside its unfamiliar world. Upon arrival, the two encounter hostile slave traders and barely escape with the help of Tamahome—a powerful young man bearing a Chinese symbol on his forehead. But, a moment later, the red light returns and takes Yui away. Desperate to reunite with her companion, Miaka asks Tamahome for assistance. However, the situation escalates when the pair encounters the land's emperor, Hotohori, who believes Miaka is the foretold priestess of the kingdom's protector god Suzaku. By gathering the god's seven Celestial Warriors, the priestess can summon Suzaku and have all her wishes granted. Hotohori hopes this will save his country, and since it appears to be a fitting solution to the girl's problems as well, he convinces her to accept the role. Meanwhile, at the library, Yui realizes she has been brought back alone. Unable to intervene, she helplessly witnesses Miaka traversing through courageous trials as the mysterious book's heroine.