What is Rogue One: A Star Wars Story is all about?

I am a huge Star Wars fan and I dig up and know almost all the news about new movies. Last days these news were: Michael Giacchino replacing Alexander Desplat and of cause a lot of noise around the reshoots which is ridiculous for me. Because if making such a big project like Star Wars you feel a great responsibility for the quality and you will check out every little part of it to create a perfect movie so this is why they have done these scheduled reshoots.

Anyway there were a lot of mentions of Rogue One last 3 month. Bad or good - doesn't matter because the movie will be great and will do awesome in the box office. They put so many resources and work into it so there is nothing to worry about.

First of all this not a saga film. This is the first spin-off movie with completely different tone and atmosphere so this is going to be more of a war film maybe even darker than the Empire. 

Right from the beggining of the marketing campaign Lucasfilm tries to seperate it from main trilogy naming the movie "A Star Wars Story", discussing about putting the opening skroll or not and using all this marketing methods to tell people not familiar with the property what it is all about. Also we are going to have Darth Vader in the movie too, so it is a huge marketing asset to push the movie forward. Despite the main villain Director Orson Krennic Vader voiced by great James Earl Jones will be a great addition to the movie. 

A great thing about it is that it takes place right before A New Hope and leads us to the events of the original trilogy. So basically it is a prequel. In the opening credits for Episode IV there is a mention of rebel spies who stole the Death Star plans during the battle. This what the movie is based of. 

How a small bunch of rebels can defeat a superpower like this on a battlefield and steal the most classified plans in the galaxy? I always wondered how could they do something like that by their heroism alone because this mission seems to be almost imposible to pull off. We are going to find this out pretty soon on December 15th, 2016.