What about this, what if they find a key in their own dimension. And then they find some sort of gateway (like under a bed, or in a fridge or whatever it would be pertaining to your characters, giant spacey wormhole anyone? Lol) and that gateway leads them to a world where their key is integral to saving the planet?
And all the gateways have this one specific symbol on it, so we all know that they are connected somehow.
As far as rating, PG-13 is basically like, violence is okay, just not like excessive gore, and some nudity, but no out-right sex scenes. You can allude to it, if you'd like, but no really hardcore graphical representations.
Sorry I've not been here quite as much as I'd like to, real-life work stuff keeps getting in the way. crying