Novelist Mikazuki Subaru has no interest in anything but books and can't stand it when people interrupt his writing work. One day, by chance, he finds a stray cat and brings it home to live with him. As he watches the cat behave in ways he didn't expect, Subaru comes up with all kinds of ideas for his latest novel. He's pleasantly surprised that the cat doesn't interrupt his work and even stimulates his imagination... but then the cat starts scattering its food around, and scratching at the door to his room. Maybe this creature really is nothing but trouble?! But the cat has its own thoughts on the matter...

Subaru has begun living with a cat as a roommate. He realizes he's run out of cat food and goes to a pet shop to buy more, but starts to panic when he's surrounded by creatures he's never been around before. Then Nana, a shop employee, starts to talk to him, asking him questions about the cat's name and age. Subaru then realizes he hadn't thought of a name for the cat yet. Nana tells him that he should give the cat a name because it's his family now, so Subaru tries to think of one, but nothing good comes to mind. As he's pondering, he comes across a book in his room called "The Moon and the Sun"...

Subaru and his editor, Kawase, plan to have their next meeting at Subaru's house. When the cat-loving Kawase sees Haru for the first time, he can't restrain his excitement as he chases her around to play with her. Meanwhile, Haru has been behaving strangely, staring at nothing and suddenly leaping around the room for no apparent reason. On top of that, she makes a mess of Subaru's parents' room, which he hadn't entered in a long time, and ends up covered in dust. Subaru has just about had enough... but then he discovers the albums in the dusty room.

Subaru is at home doing his favorite thing, reading... but then Hiroto and Kawase show up, robbing him of his time alone. Then Kawase suddenly orders Subaru to buy a collar for Haru immediately, saying it'll help identify her in the event that she gets lost. So Subaru heads for the pet shop, where Nana helps him find a suitable collar. But Nana also tells him that many cats dislike collars and only end up stressed out by them. Will Haru approve of her new collar?

The distance between Subaru and Haru is slowly shrinking. Since the vet said Haru was slightly overweight, Nana visits Subaru's home to share her advice on cat diets. Subaru enthusiastically listens to all kinds of tips he'd never heard before on correct feeding amounts, a trick to feed using a drink bottle, and more. Haru seems to enjoy this new game, as well. Eventually, Nana takes her leave, but she forgets her house key, so Subaru hurries to return it to her...