On a snowy Christmas Eve, a young girl named Atsuko hears a noise outside her family's home. Her father opens the door to find a tiny tiger-striped kitten clinging to the second-floor railing, shivering in the cold. The family brings the kitten inside and offers it some fish. The kitten bows its head in thanks before eating. As they try out names for the new arrival, Atsuko's father calls out "Mī," and the kitten responds with a meow. From that moment, the cat is named Mī-kun and becomes a cherished member of the household. This meeting marks the beginning of a fourteen-year bond between Mī-kun and the family.

The story unfolds in a quietly changing residential neighborhood modeled after the Oizumigakuen area of Nerima, Tokyo, where the creator once lived. The first part traces Mī-kun's entire life with the family, from his arrival to his final days. Subsequent parts shift focus to the various cats Mī-kun encounters during his life: strays, other pets, and the cats that drift in and out of the neighborhood. The cats communicate among themselves in their own language, which to most humans sounds like ordinary meowing, though Atsuko's father seems to understand them.

Mī-kun is the gentle, intelligent, and dignified tiger-striped cat at the center of the story. Atsuko is the girl who grows up alongside him, and her family includes her parents and, in earlier versions, two younger brothers. The neighborhood is home to a cast of stray and owned cats, each with their own personality and circumstances. Among them are cats abandoned by their owners, cats left behind when families move away, and cats struggling to survive on their own.

Each narrative arc follows a particular cat's story, often ending in sadness. A cat is left behind when its owners vanish. A stray forms a brief, fragile connection with a human before disappearing. Mī-kun himself witnesses the departures, losses, and quiet tragedies of the feline world around him. The stories are grounded in real experiences: the creator's own cat of fourteen years served as Mī-kun's model, and episodes in the manga reflect actual events from his life.

The tone throughout is tender and melancholic, balancing small moments of warmth and companionship with the unavoidable sorrow of separation and death. The manga does not romanticize the lives of cats but instead presents their joys and griefs plainly, from the perspective of a cat who watches the world change around him. Mī-kun's own journey, from a lost kitten to an old cat who has outlived many of his companions, forms the quiet emotional backbone of the work.