The story begins with the climax of the battle against the Demon Lord Yurisha. Freed, a 42-year-old veteran hero wielding the magic-interfering Divine Weapon 'Gram', fights alongside his beloved disciple, Lyle, who uses the fiery greatsword 'Laevateinn'. After Freed neutralizes Yurisha's magic, Lyle delivers a devastating blow. However, in a shocking twist, Lyle betrays Freed, stabbing him in the back. Lyle, driven by a desire for a more dramatic heroic narrative, engulfs both Freed and the defeated Demon Lord in flames, seemingly killing them both. Freed dies in disbelief, reflecting on his disillusionment with the corrupt human hero society and his deep affection for Lyle, whom he considered a son.
Freed awakens in a luxurious room within a demon castle, confused and injured. He is greeted by Stella D. Azelvain, a beautiful, silver-haired demon girl and one of the Seven Great Demon Generals, who addresses him as "Demon Lord-sama." Stella explains that Yurisha's unique resurrection ability, combined with Freed's 'Gram' interfering, caused a malfunction, transferring the Demon Lord's power and title to Freed. He now bears a red hexagram mark and possesses immense magical power, far exceeding any previous Demon Lord.
Initially, Freed rejects his new identity, asserting his role as a human hero. However, as human heroes launch an attack on the castle, Stella pleads with him to save the Demon Realm, which is on the brink of collapse. Freed, disgusted by the invading heroes' cruel, game-like slaughter of demons, intervenes. Using his newfound power, he effortlessly obliterates the heroes with a basic 'Fire' spell that levels mountains. This act of protection, ironically, rekindles his sense of purpose, making him realize he can be a "hero" by protecting the vulnerable, regardless of their race. Stella provides him with a mask to conceal his human identity from both humans and demons.
Freed, now embracing his role, begins to consolidate the Demon Realm's defenses. He clears the five invasion routes opened by the human heroes, personally confronting and defeating powerful heroes like the warrior Jade and the cruel 'Bloody Claudio'. He uses his overwhelming power to banish the less malicious heroes and ruthlessly eliminates those who revel in torture and death, repairing the magical barriers that protect the Demon Realm. During these missions, his bond with Stella, who acts as his loyal and capable aide, deepens, and he experiences genuine warmth from the grateful demon soldiers, particularly Lilim, the cheerful captain of the castle guard.
Back at the castle, Freed begins to tackle the administrative duties of a Demon Lord, aided by Stella's efficiency. However, he soon uncovers an internal conspiracy: a faction of beast-kin demons, loyal to the ambitious Demon General Zegart (a former Demon Lord candidate), were responsible for weakening the castle's defenses to facilitate Yurisha's defeat, hoping Zegart would become the new Demon Lord. Before Freed can interrogate the captured conspirators, another Demon General, the skeletal Undead King Regal, returns from the human world. Regal, a cold and pragmatic warrior, immediately executes the conspirators, preventing their testimony, and then challenges Freed, questioning his worthiness and "softness" as a ruler compared to the previous Demon Lord. The story concludes with Freed preparing to face Regal's test, now navigating both external human threats and internal demonic power struggles.
