The main character is Cockroa-chan (officially her name is G-buri-chan, G-buri being a shortened version of the word for cockroach in Japanese). The book begins with G-buri: she’s naïve and overly optimistic, eats dust balls and stray strands of hair and can't resist human food. A new tenant moves in due to cheap rent (apparently because there are tons of pests in the house). G-buri is always thinking/hoping that the new tenant that moved into the apartment is finally going to be nice, but in reality, the tenant is constantly trying to kill her with sprays, poisoned baits, rolled up newspapers, etc. Even after “protecting” the tenant from an armed robber (who fled, apparently he has some phobia of roaches), the tenant still tried to kill G-buri with some poison bait. When that failed, the tenant gave up trying to kill her, at least just for that day (which she interpreted, quite erroneously, as some kind of affection).
There are a ton of unrelated or partially related side stories involving anthropomorphized ants, bees, mosquitoes, cicadas, crickets, centipedes and various other insects. Humans faces are never shown, just hands, feet which are giant compared to the insects. Humans are portrayed as mostly aloof to the world of insects and sometimes even sadistic (one human baits an ant to fall into an antlion nest).
The book wraps up with G-buri’s story. The tenant moves out and she’s alone again, with only one strand of the tenant’s hair as a reminder. Then one day, another potential tenant is looking at the apartment and it ends with G-buri again being naively excited about meeting a new human.