Fat Cat of Underwhere by Bruce Hale

Cats are not meant to save the world.

Fitz the cat is supposed to be enjoying the normal activities savored by those of the feline persuasion—napping (a lot), nibbling on house plants, sharpening his claws on expensive furniture, and, most importantly, never, ever doing what a human wants him to.

But instead, thanks to Zeke, Stephanie, and Hector—those meddling, tuna-hatin', whiskerless kids who dragged him down to Underwhere—Fitz has bigger fish to fry (though, of course, he prefers his fish raw).

Not only has he started thinking like a human, he's actually helping them recover the stolen Scepter of Underwhere, battle a wild pack of triceradoodles, outwit a roaming band of savage mice, and foil the plans of the strange new movie director in town, who happens to smell awfully familiar. . . .