The protagonist, having lived countless lives, finds herself in a void between reincarnations, a familiar experience after her millionth rebirth. This time, however, her usual transition is interrupted by a peculiar, pink-haired, winged child who gifts her a glowing orb that merges with her body, initiating her next reincarnation.
She awakens in a new life as an infant, cradled by a woman named Diana, who claims to be a manifestation of the protagonist's own memories and knowledge. Diana reveals that the current world is on the brink of destruction, threatened by two immense entities, "Heavenly King Caldes" and "Demon King Mizel," who are slowly consuming the continent. Their "curses" cause children to die at one year old and adults at forty, decimating the world's leadership and knowledge base. Diana, having absorbed the protagonist's vast accumulated knowledge, is a being of immense understanding but lacks personal memories. She explains that she was intentionally placed within the protagonist by the child from the void, and her existence is tied to the protagonist's.
The protagonist, now named Sanaria, is born into the Holy Kingdom of Revelrad, a religious state. Her birth is a momentous occasion, as she is the "last child of humanity" and the only one seemingly immune to the curses. Her mother, the previous priestess, died in childbirth as a sacrifice to pass on the "miracle" of immunity. Sanaria's grandfather, Pope Kyles, a man burdened by the world's impending doom and the loss of his daughter, takes on the responsibility of raising her. He is aware of Diana's presence within Sanaria, recognizing her as an anomaly.
Sanaria, possessing the protagonist's accumulated knowledge and skills from her past lives, is exceptionally intelligent and driven. She undergoes rigorous training, essentially a deadly obstacle course designed by Diana and approved by Kyles, to prepare her for the world's challenges. Her unique abilities, including "Infinite Reincarnation," "Integrated Unconsciousness" (allowing her to think with multiple minds simultaneously), and "Area Change," are evident from a young age.
At three years old, Sanaria, accompanied by Anna, a struggling but determined Holy Knight trainee, ventures out for her first monster hunt. This marks her initial step into the world beyond the castle walls and a crucial part of her training. She demonstrates an extraordinary aptitude for combat, dispatching monsters with precise, efficient techniques that far surpass her age and apparent skill level. She also begins to explore the world's magic system, discovering that it operates on principles beyond the conventional understanding, and starts teaching Anna a more effective method of spellcasting.
Sanaria's primary goal is to break her reincarnation curse, but she also recognizes the dire state of the world and feels a responsibility to help save it. She learns about the world's history, the origins of the curses, and the ongoing war against Caldes and Mizel. She also discovers that other races, like elves and dwarves, have largely disappeared, leaving humanity isolated.
Her adventures continue as she trains, learns, and interacts with the people of Revelrad, including the stoic Holy Knight Captain Mariaani, who is privy to some of Sanaria's secrets. Sanaria's unusual abilities and rapid growth attract attention, both positive and suspicious, as she navigates a world teetering on the edge of annihilation, driven by her past lives' experiences and the need to forge a new future.
