The third season finds Tokyo alight, as a mysterious affliction of spontaneously combusting rips through the city's population. Enter the Fire Force, with Shinra now part of Company 8 and prepared to save the city from oblivion — if he can keep his own secrets at bay.

Despite standing at the zenith as the strongest exorcist, Haruyoshi Kuga sought even greater power, which led to his demise at the hands of those who envied his might. As he draws his last breath, he casts a spell that allows him to reincarnate and swears to become more cunning, hoping that all his efforts will prove useful in his next life. Haruyoshi is soon reborn in another world as Seika Lamprogue, an illegitimate son of a noble family that prides itself on magic. However, his new family largely ignores him due to his lineage and magic deficiency. Nevertheless, the unbothered Seika leverages the knowledge from his previous life to make numerous valuable allies. As he begins to explore the true limits of his abilities, Seika must also be wary of the consequences that tarnished his past and avoid repeating the same mistakes.

To prepare for reincarnation in another world, Kelvin gives up select memories from his previous life in exchange for powerful abilities, additional skill points, and an S-class summoner title. As a bonus, the goddess facilitating his rebirth, Melfina, offers him a choice of any companion to give him a head start in his summoner role. Kelvin—who has fallen head over heels for Melfina at first sight—promptly chooses the deity, confident that his passionate feelings for her will resurface even without all of his memories. Kelvin embarks on his exciting new journey with Melfina as his guide. However, in order to summon his beloved goddess' physical form, he needs to acquire significant amounts of mana points—and the best way to accomplish this is to level up by fighting strong enemies and making contracts with even stronger companions.

En route to conquer a Great Labyrinth with his allies and former classmates, Hajime Nagumo is sidetracked by a tragedy that befalls his party member Shea Haulia. After encountering a group of rabbitmen caught in an ambush, Nagumo learns that members of Shea's Haulia Tribe have been abducted by soldiers of the Hoelscher Empire. To alleviate the anxiety of Shea, whose father may be in mortal danger, Nagumo decides to head toward the empire and rescue the missing Haulias. After arriving at his destination, Hajime is caught up in political turmoil between the empire and the Heiligh Kingdom. As princess Liliana S. B. Heiligh tries to formalize an alliance with the empire to protect her weakened kingdom against demon attacks, the Haulia Tribe plans to assassinate Emperor Gahard D. Hoelscher to free all rabbitmen from the empire's threat. Determined to protect his allies at any cost, Hajime will need to ignore all political considerations and unleash his unstoppable power on anyone who may stand in his way.