An animal wandered into your home and decided it wasn't going to leave. As the days pass, you get the oddest feeling you know this animal, except as a human. Eventually it's impossible to deny this animal was someone famous.
Do they begin speaking? Do they want to reclaim their previous fame? Do they have unfinished business?
It can be a famous person from this world, from the world your characters are in, or from a story brought into the mundane here-and-now world.