In the land of the cat people, it is a serious crime for outsiders to enter without permission. Despite this, Nene, a small mouse-girl, has secretly immigrated from Ninoshima, an island inhabited only by mice, to the country of cats she has always dreamed of. Disguised with false pointy ears and a fake tail, Nene is determined to pretend to be a cat in order to accomplish a specific, mysterious goal.

The story begins as Nene infiltrates this forbidden feline nation and finds work at the Rai Rai Teahouse, a refined establishment known for its beautiful cat-girl employees. There, she becomes the eleventh cat working at the cafe, joining the ten existing lady cats. Living and working alongside these sharp, sexy, and sometimes mean cat-girls, Nene begins a dangerous undercover life where she must struggle to learn how to be a cat from her senpais while keeping her true rodent identity a secret.

The setting is the kingdom of the cat people, a closed-off realm where Nene must navigate the constant risk of exposure as an illegal outsider. As she works at the teahouse, her daily life is a series of challenges, striving to mimic feline mannerisms to avoid being discovered. While the initial chapters focus on her struggling to adapt and work alongside her cat coworkers, later narrative arcs explore deeper mysteries and threats to her disguise. The story progresses into arcs such as Nene attempting to avoid a feline vaccine, which could potentially reveal her true species, and a two-part night market arc where she partners with a liar. A journey to her homeland, Mice Island Nostalgia, further explores her past and the reasons she left. Throughout these adventures, Nene must protect her secret while pursuing the true objective that drove her to risk everything by entering the country illegally.