I say mostly believable, because some of the story elements are tremendously campy with various characters acting in almost cartoonish ways--which is a bit jarring considering the otherwise grim depictions given of the world as it was approaching its apocalypse. I was also particularly bothered by the villain whose head (perhaps symbolically) was noticeably too big for his body. I'm also a little concerned that this DLC--due to some of the events in its story--has essential information that players will need before the next game comes out. The DLC for the previous game (which I felt was better in terms of story and game design changes) was not as essential and was thus more forgivable as an add-on. This should have been part of the main game from the start.
With those issues aside, I loved playing the game. I particularly like using a flying mount (who gets a great upgrade) through most of the game, and I hope that the next game allows its use from the beginning. This DLC used clever ways of limiting the flight ability from time to time which were believable for both the story and the game mechanics. Basically, this DLC makes a great game even better.