The story begins by establishing a world where various races, including vampires, beastmen, fairies, angels, demons, elves, dwarves, and humans, once engaged in a devastating Species War to determine dominance. This conflict, fueled by the unique "species traits" of each race, nearly led to extinction, ultimately ending in a fragile peace about a century ago. In the present day, the protagonist, Kale, a fifteen-year-old human, lives a life far from peaceful. He is a "failure" because his species trait has not yet manifested, making him a target of bullying by his classmates at Heilia Academy.
Kale's younger sister, Muir, is a renowned and exceptionally skilled swordswoman known as the "Sword Princess." She is also a prodigy in the Guild, an organization that brokers work. Muir financially supports Kale and their household, as their parents mysteriously disappeared years ago. Despite his lack of abilities, Kale insists on attending the academy, partly to honor his sister's efforts and partly because he believes education is important.
One evening, while taking a walk, Kale stumbles upon a scene of unconscious people in an alley. Before he can react, he is attacked by a young, silver-haired vampire girl named Clena. She bites him, intending to make him her familiar, a human subservient to an inhuman. Kale's blood is drained, and then Clena injects him with her own blood, initiating the familiarization process. To his surprise, Kale immediately gains enhanced physical abilities and the ability to manipulate blood, a vampire's species trait. He discovers he can use this power with surprising ease, even performing advanced techniques like "Blood Dance."
However, their encounter is interrupted by a demon man, Alguard, who is hunting Clena. Kale, now possessing vampire-like speed and reflexes, manages to escape Alguard. Clena, despite being a pure-blooded vampire, is injured and unable to defeat him alone. She urges Kale to assist her, revealing that he can use blood manipulation. To her astonishment, Kale, despite being human and having never manifested any ability before, can use these powers effectively. He uses his newfound abilities to defeat Alguard, though he is confused by the ease with which he wields these powers. Clena, impressed and intrigued by Kale's unusual aptitude, asks him to become her permanent familiar and bodyguard, as she is being pursued by the Alguard Empire for her royal blood, which is needed for a special weapon. Kale, initially hesitant due to the danger and his desire for self-reliance, eventually agrees after Clena offers substantial financial compensation, which he hopes will alleviate his sister's burden.
The next day at school, Kale's unusual strength from his brief familiarization with Clena is evident when he easily defeats a bully named Lawrence in a mock combat. Lawrence, a noble, is enraged by Kale's unexpected power and Clena's defense of him. Clena, a noble herself, reveals she is on the run from the Alguard Empire, which is seeking her royal blood for a secret weapon project. She enrolls in Heilia Academy, hoping to blend in, but is quickly discovered.
Kale and Clena decide to join the Guild, "Tenmei Brigade," to gain access to its facilities and potentially find a safe haven. To do so, Kale must register as a vampire, as his human abilities are insufficient. He undergoes a test with Clena, where they both demonstrate exceptional blood control, far exceeding the normal limits for vampires. The Guild Master, Gallia, is impressed by their "rank" and allows them to join. They are assigned a high-point quest to destroy a goblin nest, which they accept with the help of two other Guild members, Doros and Pixis. They are soon joined by Aina, a powerful beastman who previously made Kale her familiar for a short time. Aina, now an A-rank member of the Guild, offers her assistance, creating a party of five.
Meanwhile, Muir, Kale's sister, is revealed to be a highly influential figure, with the Guild preparing extensive support for her rumored brother, leading to many fakes attempting to claim the title. Kale and Clena, using aliases, are accepted into the Guild. The story ends with Kale and Clena embarking on their first Guild mission together, with the underlying threat of the Alguard Empire and the mystery of Kale's extraordinary abilities as a human familiar.
