In the 21st century, humanity's love for cats grew so deep that nearly everyone owns one as a pet. However, selective breeding of cats results in an extraordinary individual who carries a strange virus. The outbreak—dubbed the Nyandemic—turns any human who makes contact with an infected feline into one themselves.

Kunagi, a man who has lost all of his memories, is forced to flee mid-shift from a cat cafe. With him are siblings who run the place, Gaku and Kaoru, and a customer named Tanishi. But Gaku confesses that he is already infected and pleads with Kunagi to protect his sister. After experiencing the danger firsthand, the remaining three begin searching for a way to survive the very animals they adore.

Despite the government's best efforts to halt the feline hordes, the Nyandemic uncontrollably spreads. Human extinction seems more probable by the hour—but Kunagi, Kaoru, and Tanishi refuse to surrender their humanity without a fight!