The protagonist, a reincarnated Japanese woman named Mai Fuyou, finds herself reborn as an infant named Scarlett Northmark in a world vastly different from her previous life. Initially disoriented and unable to communicate, Scarlett gradually regains her cognitive abilities, realizing her transmigration. Her new family appears to be wealthy, with multiple servants attending to her needs. Scarlett's primary goal is to understand her new environment, but her infant body limits her information gathering.
As Scarlett grows, she experiences the indignities of infancy, particularly diaper changes, which challenge her sense of dignity. A significant discovery occurs when she notices a pervasive black, misty substance clinging to everything, including herself. This "Moyamo-ya" is invisible and bothersome to her, prompting her to clean it off whenever possible, though others seem oblivious to its presence. She deduces that this is a unique characteristic of this world and resolves to eradicate it.
Around six months old, Scarlett's vision improves, allowing her to recognize faces and understand some of the language. She learns her family's names and some basic greetings. Her father, though rarely present, is doting, and her mother is exceptionally beautiful. A significant turning point arrives with her first birthday when she receives a small toy broom. To her surprise, wielding this broom allows her to easily make the "Moyamo-ya" disappear, leading her to believe it might be a magical item or that she possesses magical abilities. This discovery excites her, and she begins using the broom to clean her surroundings, much to the bewilderment of the servants, particularly the young maid-in-training, Fran.
The "Moyamo-ya" cleaning spree leads to a dramatic event two weeks after her birthday. Scarlett wakes up to find her entire bed and surrounding area covered in an unprecedented amount of the black substance. This overwhelming amount causes her to cry uncontrollably, revealing the "Moyamo-ya" to the terrified servants. It becomes clear that while the servants cannot see the "Moyamo-ya," Scarlett can, and the massive accumulation appears to originate from her. She realizes she is somehow absorbing and accumulating this substance.
Scarlett experiments and discovers that by concentrating, she can prevent the "Moyamo-ya" from leaking out of her body, and by further concentrating, she can solidify it. This process, however, results in the formation of a beautiful, multi-colored gem-like sphere instead of the black substance she expected. This creation causes a stir, as the gem is deemed incredibly valuable, leading to a family discussion and a subsequent cover-up. Scarlett concludes that the "Moyamo-ya" is likely her own excretion, and the gems are a byproduct of her body's transformation of it. She devises a plan to transfer the "Moyamo-ya" into inanimate objects, effectively cleaning herself and preventing future overflows, though the process of creating gems is still a mystery.
