Kenichi Yamada has spent most of his adult life chasing the next punishing challenge in online gaming, only to find the modern landscape dominated by forgiving mechanics. As if fate were listening to his desire, Kenichi receives a sudden, mysterious invitation to an untitled game promising an inconceivable level of difficulty. Delighted, he selects its most ruthless setting without hesitation—Hell Mode. However, instead of diving into another virtual game, Kenichi is reborn in another world as a peasant named Allen, forced to grapple with a cruelly complex system where even the basics of progression demand relentless effort. Consequently, Allen must build his strength from the ground up and learn to master the summoner class. With his resilience being tested at every turn, Allen embarks on an uncharted journey that might just prove to burn hotter than actual hell.

After Jinta and Suzune ran away from their home as children, they were taken in by Motoharu—the virtuous sentinel of Kadono Village—and his daughter, Shirayuki. The settlement was ruled by its shrine maiden, Itsukihime, whom only a select few were allowed to interact with. When Itsukihime passed away, Shirayuki was appointed in her place. Wishing to stay in contact with her, Jinta worked diligently for years and was finally selected as the next sentinel. As the protector of the village, Jinta's duty is to eliminate any threats. One day, he encounters a demon in a nearby forest. At the end of the battle, the demon makes a proclamation about the destined ruler of all demonkind. He attaches his severed arm to Jinta, turning him into an unaging demon man. By the time Jinta comes to, Kadono is on fire. Suzune has transformed into the demon from the divination, and due to a series of misunderstandings, she murders Shirayuki and flees. Devastated, Jinta cannot forgive Suzune for her actions; he departs on a journey to find her. As Jinta travels around Japan across the eras, he endeavors to protect as many people as possible from aggressive demons. In his quiet moments, he contemplates what he should do when he once again faces his childhood friend.

Bestowed with magic from God, witches worked to guide humanity toward progress, garnering adoration and fear alike from humans. However, times have since changed. With the advancement of scientific knowledge, humans now stand on an equal footing with witches. Determined to weave their own destinies and establish supremacy over the world, the mighty Redia Empire has ordered the extermination of all witches, which may bring the era of magic to an end. Fleeing from the decree, the famed witch Chloe and her apprentice, Adonis, seek refuge. Hot on their heels, the Empire captures the duo and mercilessly executes Chloe. Agonized by the brutal murder of his master and the callousness of humans, Adonis vows to avenge Chloe by annihilating humanity with the very magic they intend to destroy.

Right as he is about to earn a promotion, a salaryman meets an untimely demise. Taking pity, a mysterious being grants him a second life in the land of Galdardia as a teenager. Choosing the name Luciel for himself and a role as a healer, the young man sets off for the nearby town of Merratoni. There, he learns that healers are a coveted resource, yet they often take advantage of others for monetary gain. Vowing to be different from other healers, he joins the local branch of the Healers' Guild and begins unlocking his magical potential. After failing to save someone with his magic, Luciel is determined to get stronger. To improve his physical strength and hone his magic skills, he registers as an adventurer and takes rigorous training under Brod, the chief instructor of the Adventurers' Guild. Soon, Luciel gradually finds his way through Galdardia, gaining a reputation as a selfless healer who aids others regardless of expense.