Since ancient times, the citizens of Slantania have lived within a shroud of miasma, a force that brings forth monsters creating havoc around the world. When the kingdom's Order of Knights becomes incapable of containing the monsters, the Grand Magus summons a holy maiden—the legendary Saint—to obliterate the horrid creatures and restore peace. One evening, as Sei Takanashi returns home from a long day at work, a magical force pulls her into a foreign world. Upon her arrival to the kingdom of Slantania, she overhears that the Grand Magus inadvertently summoned two Saints—Sei and a brown-haired girl. Needing only one holy maiden, the prince declares the other girl as the Saint, leaving Sei to her own devices. Sei sets her sight on the Medicinal Flora Research Institute, an establishment known for its studies regarding herbs and potions. While indulging in her latest passion, Sei has a fateful encounter with the commander of the Third Order of Knights. But little does she know, her aptitude as a Saint will continue to exert its influence over her new life.

When 20-year-old college student Haruto Amakawa dies in a traffic accident, he does not expect to wake up in an unfamiliar world in the body of a young boy named Rio. As their memories and personas fuse, Rio realizes that he now also possesses magical powers. He is relieved to find that his burning passion for revenge against his mother's murderers has not subsided, despite his newly changed identity. Not soon after, Rio comes across the kidnapped princess of the Bertram Kingdom and saves her without hesitation. To express his gratitude, the king grants him the opportunity to enroll in the Bertram Royal Academy. Believing this to be a new chapter in his life, he is excited to study at this prestigious academy, but life here proves to be difficult for him, a slum-dweller surrounded by the majestic children of nobles.

The world's greatest assassin had sworn lifelong allegiance to the organization that raised him. However, despite his loyalty, that very same organization takes action to silence him, ultimately leading to his demise. Drowning in frustration and regrets he can no longer suppress, he finds himself in an audience with a goddess attracted by his exceptional skills. The goddess offers him reincarnation into a magnificent world of swords and magic so he can perform a crucial mission: prevent that world's destruction by slaying its hero. Accepting the goddess' request, he is reborn as Lugh Tuatha Dé, the son of a noble family of assassins serving the Alvan Kingdom. Under the guidance of his father, Lugh learns new assassination techniques that significantly differ from the cold-blooded and unsympathetic killing style of his previous life. Furthermore, his other talents bloom, allowing him to meet new allies and acquaintances. Even so, Lugh knows that his efforts are far from adequate, because a monumental adversary such as the hero can only be defeated with perfection.