To add on to this, don't even share this story if you've written a few chapters of it, that's just giving the thieves more good stuff to steal from. Keep it all hidden away, the more you write, the more they can steal. Completed the entire 8 book series with detailed world-building and fully realized characters? Lock that up! You're just asking for people to take all your great ideas for themselves!
Actually, scratch all of that. No matter how secure your vault is, writing down anything just creates too big a risk for theft. Keep all of your ideas locked away safely in your mind and take it with you to your grave.