First of all two words - magical mice. Why? Because "magical mice" is just fun to say...
Okay, so, for a plot...
He's still young, and doesn't know it yet, but the kitten is actually The Golden Cat, a prophesied cat hero who has his own quest, and will save the world for all catkind. There will be a string of events throughout the campaign that the party will thoroughly misunderstand, where the cat's adventures impinge upon the human world.
Just like a human hero, along the way he'll pick up his own companions that start showing up and hanging around (these may include any mounts or Animal Companions the PCs may have - they think they have Human Companions...). You get the idea. One always talks about how the PCs are not the only adventuring party in the game world, right? Well, in a magical world, why do they all have to be human?
Read Tad Williams' Tailchaser's Song, or Gabriel King's The Wild Road and The Golden Cat for references.