Stannis Baratheon attacks King's Landing! That's the entire storyline this episode. We don't go to the Wall, we don't go to Qarth, we don't check in on Arya or Theon or anyone. If you're not in King's Landing or attacking it, you don't get to be in this episode. The overall attack is stopped, thanks to Tyrion's boat full of wildfire and a timely speech that comes right after Joffrey abandons the defense of the city.