From Crunchyroll:
We are the scum of humanity. Hanabi and Mugi are bathed in envious gazes from all around as a beautiful couple with excellent moral conduct. But these two apparently perfect people in an apparently perfect relationship share a secret they can't tell anyone.

Included One-shots:
Volume 1: Nyan Nyan Prelude
Volume 2: Nyan Nyan Serenade
Volume 3: Nyan Nyan Oratorio
The lonesome struggles of the only living being on Earth.
Volume 4: Nyan Nyan Cantata
A pet cat looks back on her life with her kind master when she feels that her days are coming to an end.
Volume 5: Nyan Nyan Andante
An abandoned cat grows fond of her new owner but she starts to feel insecure when the kind-hearted owner adopts another stray creature.