The story begins with a dream sequence depicting a tragic battle between two brothers, Tsurugi and Jin. Tsurugi, the younger brother, is forced to kill Jin, his older brother, who was an enemy. Jin, before dying, makes Tsurugi promise to defeat a specific individual, implying a larger conflict. This dream is revealed to be a memory from someone else's past, possibly a viewer of the events.
The protagonist awakens to find himself as a baby, realizing he has been reincarnated. He deduces this is the world of his favorite manga, "Kemono Uta" (Beast Song), a battle manga with a dark fantasy theme. His fear is confirmed when he learns his name is Izayoi Jin, and his parents are Subaru and Rin. He is horrified to discover he has reincarnated as the older brother, Izayoi Yaiba, a character destined to fall into darkness and a pivotal antagonist. His primary motivation becomes avoiding this tragic fate and living a peaceful life.
As Jin (now Yaiba) grows, he develops the ability to walk at an unusually young age, impressing his parents. He also begins to train his spiritual power, aiming to master ice and wood-based magic, which are compatible with his innate talents. He learns that his younger sibling, Tsurugi, is actually a girl named Tsurugi, which further complicates his understanding of the original story. Yaiba discovers his own unique talent: the ability to "descend" or act as a vessel for spirits, particularly those related to deities.
A significant turning point occurs when his grandmother, Suiou, a powerful diviner, detects Yaiba's abnormal spiritual power and his potential to become a "singularity." She reveals that Yaiba has inherited the "descending" talent, which the Kureki clan has long sought. Suiou decides to train him to harness this power, believing it necessary for his survival, as he is also being targeted by fallen gods. During a ritual, a divine spirit named Shinki appears and claims Yaiba as her "destined person," hinting at a future connection and potential danger.
Later, Yaiba, now six years old, continues his training under his father, Subaru, a skilled fighter. He demonstrates remarkable proficiency in ice magic, creating complex ice sculptures like dragons. During this time, a significant plot point is revealed: the sword "Shiki," a divine artifact capable of manipulating the seasons, has gone missing from the treasure vault. The Utsuki clan, specifically the patriarch Ouma and his daughter Ryuka (a key heroine from the manga), arrive investigating the sword's disappearance, as its energy signature has been detected near their home. Ouma, a powerful fighter and friend of Subaru, notices Yaiba's unusual strength and the presence of Shiki's aura around him, suspecting Yaiba is connected to the sword.
The narrative then shifts to Ryuka, a young girl with deer-like horns and serpentine scales, who is bored with her life. Her father, Ouma, leaves on a mission related to the sword and increased miasma. Yaiba, meanwhile, is confronted by Shinki, who has taken a liking to him and desires his name. He refuses to reveal it, fearing its misuse. He also expresses his desire to become stronger to avoid his predetermined dark fate and protect his family.
The situation escalates when a powerful miasma appears, and creatures known as "kemono" begin to emerge. Yaiba's family and Ouma's subordinates fall into an unnatural sleep, surrounded by black butterflies. Yaiba discovers Ouma is injured and fighting a mysterious girl named Kochou, who claims to be a newly born kemono and has a connection to the butterflies. Kochou reveals she has been drawn to Yaiba and, after a brief struggle where she isolates Yaiba with a barrier, bites his neck, implying a deeper, possibly parasitic, connection. The story ends with Yaiba facing this immediate threat and the growing realization of the dangers he and his family face.
