Onee-san to Neko wo Kau summary: One day, on his way home from school, a boy is passing through the park as usual, when he's asked, "Wanna sniff my pussy?" It's a mysterious older lady and the cat she's sniffing. When he hears she's apparently homeless and living in the park... he feels sympathetic and brings the woman and cat home, where his grandmother tells him, "Well, you picked them up, so you better take responsibility." So begins the boisterous and somewhat ecchi daily life of a boy, an older woman, and cats.