Articy:Draft is a game design/narrative/story writing tool that is uniquely suited for planning your game stories - and it is now on sale at Bundle Stars for only $5.49 this week.
I use Articy:Draft myself to plan and write my adventure stories and character dialogues. This is not the newest version (3 was released in February), but the main difference between v2 and v3 are expanded export options and Unity integration. You also get 35% off till April 16 if you want to upgrade to the latest version (around$60?)
There is nothing like it on the market. If you are at all interested in making good back stories and narratives for your games this is the tool to have. Used by game designers and writers at some very large game studios!
Sorry for the shameful plug, but it really is a steal and fun to work with to plan out your game stories and design.