Ok, the second question is easier, actually, the fragment in the description pretty much tells the story. But in short, a pony broke a law for Twilight, a very important law and he was sentenced to death. She volunteered to share part of the sentence that he won't be executed but instead they will both be whipped.
As for the story... that's a hard question, because a short summary would never give justice to the actual complexity of the plot here.
Basically, Equestria we know from the show is a lie carefully sustained by Celestia to keep ponies happy and innocent, while other ponies that live outside of her realm are fighting in a war to protect it. Her younger brother, prince Terrato (seen on this picture) is leading them to fight against their natural enemies. Twilight accidentally discovers the truth and decides to join the fight to show Celestia how wrong that division is. That's the setting, the story is much more complex, because it shows how characters are changed by the new reality they find themselves in, how their friendships are challenged by other things, like duty, love, jalousie or ambition and how fighting against monsters one has to be very careful not to become a monster himself. There's also a huge amount of world building and expanding on the lore from the show to incorporate all sorts of things. The thing is, the war doesn't really play the main part, especially in the second part of the series. Characters and their individual storied are the main focus.
But like I said, I can't describe 90+ chapters just like that
and if you don't want to read it, there's a brilliant audio version on youtube here:
[link]