One day, Guest was conquering a dungeon in another world with his familiar, the Queen Succubus Eri, to collect resources.

However, just as they reached the middle layers of the dungeon, they were suddenly attacked by a female knight.

The person identified herself as Hakua, a Holy Knight serving the interventionist god Fuonberg of the Light faction. Branning the succubus and Guest—who was traveling with her—as villains of the Dark faction, she attacked without listening to a word.

However, Guest is the First Apostle of Rem, the neutral goddess of the Harem. He is far stronger than the Heroes or Demon Kings of this world. He easily repelled and neutralized the lone Holy Knight with his bare hands.

After being defeated so easily, Hakua immediately turned into a coward, desperately begging for her life. She was the type to be weak against the strong and arrogant toward the weak.

Disgusted by Hakua's pathetic attitude, Eri performed Succubus Magic, embedding a Succubus Core directly into Hakua's soul.

With her soul turned into a succubus, Hakua's body begins to transform from human to succubus...

The plan is to make Hakua, who views succubi as pure evil, fall into being one herself and put her in her place.