In the mystical world of Seal Fall, eleven-year-old Ryya Takerayashi lives a serene life in the forest, accompanied only by his remarkable collection of over a thousand slimes. Each slime, nurtured with various foods, evolves into rare forms with unique abilities, such as the Cleaner Slime, which washes his dishes, and the Scavenger Slime, which consumes garbage. Despite his solitary existence, Ryya’s world is disrupted when he encounters a group of knights with an injured companion.

Wishing to assist, Ryya offers his homemade medicine. His initial awkwardness in communication—due to his prolonged isolation—leads to confusion among the knights.

Nevertheless, they accept his help and follow him to his hidden residence. The knights are astonished to find a child living alone in such a remote area and are further surprised by his advanced magical capabilities and the presence of sticky slimes.

Among the knights is Reinhard Jamil, the head of a noble family, who expresses gratitude and curiosity about Ryya's background. Ryya reveals that he learned magic from his deceased grandparents and lives alone after their passing three years ago. Reinhard, concerned for Ryya's safety, decides to consult with his family about his future.

As Reinhard's team prepares to leave, Ryya feels a pang of guilt for not disclosing the truth about his past. He remembers his previous life in Japan as an overworked office worker who died from exhaustion. In the afterlife, gods introduced themselves and offered Ryya reincarnation into another world, complete with a new identity and memories. They helped him craft a backstory and advised him on his new life, with the condition of keeping his past a secret.

Reborn as an eight-year-old in Seal Fall, Ryya tests his powers and discovers the nuances of monster taming through magic. His encounter with Reinhard's family, who are also seasoned monster tamers, reveals the depth of his abilities.

Ryya's proficiency with slimes impresses them, especially when he merges his slimes to showcase their capacity and efficiency.

Reinhard’s daughter, Eliaria, eager to tame her first slime, learns from Ryya about the conditions necessary for evolving a slime into a Cleaner Slime. The knights are both shocked and amused by the unconventional requirements, and Eliaria, despite her initial hesitation, sets out to capture her slime.

Meanwhile, Ryya prepares to join the Jamil family on their journey to the city of Gimmel.

The family is fascinated by Ryya’s advanced magic skills, including his mastery over all elemental magic and alchemy. During their travel, Ryya demonstrates his problem-solving abilities by using his slimes and magic to resolve issues caused by an earthquake and heavy rain, earning the admiration of the Jamil family.

Upon arriving in Gimmel, Ryya accompanies the Jamil family to the church, where he undergoes a formal ID registration. Here, Ryya’s consciousness is briefly summoned by the gods, who express their delight at his accomplishments. They reveal that an ancestor of the Jamil family, also from Earth, had introduced monster taming to this world.

Ryya is pleased to learn of this connection and the shared legacy of his friend Eliaria.

The gods praise Ryya for his distinctiveness and enthusiasm, contrasting him with other Earth-born individuals who struggled with their past lives. They reassure him that his continued efforts are noteworthy and appreciated. As Ryya returns to his body, he receives a status update reflecting his impressive capabilities.

Back at the Jamil manor, Ryya is advised to join a guild to further hone his skills. Elise, Reinhard’s wife, warns him to be cautious, given his extraordinary talents. Eliaria expresses a desire to practice magic with Ryya, and he eagerly agrees, fostering their growing friendship.

The following day, as Ryya accompanies the Jamil family to the Tamir Guild, Eliaria signs up as a monster tamer. Despite some challenges, her determination and Ryya’s guidance ensure she embarks on her own path as a tamer. Together, they look forward to new adventures and the exploration of their magical world.

Ryya’s journey, from a reclusive child with a unique bond to his slimes to a valued member of the Jamil family, highlights his transition from isolation to a promising future filled with potential and camaraderie.