From the manga habit:

Rio is a 14-year-old boy with no particular special traits, except his burning love for books. He's awkward and considered odd at best by his peers. Rio also has a twin brother, Mao. Although they look alike, they couldn't be more different. Mao is good at everything, is popular, and has future plans to have a big, happy family. Despite their differences, the brothers are close.

One day, when Rio attempts to retrieve a cat that wandered into the street, he is hit by a car right in front of his brother's eyes. Everything fades to black. In the darkness, a voice offers a choice: to live or to die. Fading rapidly, Rio gives the obvious answer: to live!

Rio awakens in a world filled with foreigners and strange animals he's never seen before. That's not the only odd thing–Rio now has the form of a black cat! He quickly learns cats are revered in this world because of their special role as familiars of wizards.