Chapters 3 through 12 of the light novel detail the unexpected arrival of a mysterious cat named Nana and the subsequent unraveling of secrets surrounding her. The story begins with veterinarian and hospital director Kimura Suzu, a 31-year-old single woman, observing news of an approaching typhoon. Her animal hospital, Kimuraya Animal Hospital, had already moved its inpatients to the second floor in preparation for potential flooding.
On this rainy morning, Suzu discovers a remarkably beautiful and seemingly human-like cat collapsed at her doorstep. The cat, who she later learns is named Nana, is suffering from hypothermia. Nana wears a peculiar brown leather collar with a serial number and the name "Nana" engraved on it, hinting at something unusual. Upon examination, Suzu and her close friend and vice-director, Hayakawa Hitomi, are astonished by Nana's extraordinary physiology. X-rays and CT scans reveal Nana's brain is abnormally developed, resembling a miniaturized human brain, and she possesses human-like vocal cords, suggesting she can speak. They hypothesize Nana is a result of rapid mutation and evolution, or a more unsettling possibility they hesitate to consider.
Fearing for Nana's safety and recognizing her uniqueness, Suzu and Hitomi decide against reporting her to the police, opting to protect her. They discover a broken, miniature GPS tracker on Nana's collar, which Hitomi disposes of. Nana awakens soon after, exhibiting signs of retrograde amnesia, unable to recall how she arrived or why. Suzu, recognizing Nana's intelligence and ability to communicate, invites her to live with them, forming an immediate bond. Nana, despite her amnesia, displays advanced cognitive abilities, including computer literacy and color recognition, and expresses a desire to become a veterinarian, a wish that sparks amusement and curiosity in Suzu and Hitomi.
The narrative then shifts to a flashback, revealing Suzu's past. As a child, Suzu was a prodigious swimmer with Olympic aspirations. Her life took a turn when her parents adopted a kitten named Rin. Suzu became deeply attached to Rin, neglecting her swimming training. This led to a decline in her performance and a shoulder injury, causing her to abandon her swimming dreams and fall into a deep depression. During this period, Rin exhibited unusual behavior, refusing to eat unless Suzu did, and eventually died from acute stress cardiomyopathy, a tragedy that profoundly impacted Suzu and her family. This past trauma resurfaces as Suzu contemplates a significant decision she needs to make, involving Nana and the family's future.
The chapters culminate with Suzu preparing to discuss a crucial matter with her parents, Hitomi, and Nana. She reveals a cat carrier, hinting at a past connection to a cat named Rin. The story sets the stage for a significant decision that will involve Nana, suggesting a deeper purpose for her arrival and a potential connection to Suzu's past trauma.
