The protagonist of the movie, Miyo Sasaki, is a school girl who is a bit over the top, her parents are divorced and she doesn't get well with her stepmother but she likes to hide her feelings and pretends that everything for her is fine and good. So she exaggerates on everything, especially with her crush, Kento Hinode, which despises her because of her attitudes. She even receives a nickname "MUGE" (which I forgot the meaning now, but I could even say that she suffers bullying from the school from everyone except her best friend).
So one day at the summer festival a fat calico cat, The Cat Store Owner (shame that he doesn't have a name), asks her if she want to realize a wish and offers a cat faced mask. After this, Miyo gains the ability to transform into a blue eyed white kitten and she uses this opportunity to approach her crush (who like cats) and start a double life as the stray cat Taro.
So, this movie is more about the couple, Miyo and the relationship with her family and Kento than about cats (the protagonist doesn't even like cats, she just acquired the mask so she could be close to her crush because her stepmom has a cat that Miyo affirms that "doesn't like her"). However it has some cute cat moments, telling the best parts without spoiling too much, the movie has a side story with the stepmom's cat, Kinako; a magic cat place, Cat Island (which remembered me the Cat Kingdom from Neko no Ongaeshi) and a really funny/cute scene at the end of the movie with the Cat store owner.
The movie isn't the best one, the animation, art and music is nothing special, but it's funny enough to pass time and even tough the cat was "villain" nothing bad happened to him so it was just a minor detail. I give it 3/5 stars and recommend to watch if you like cats.