Intergalactic Fallout.
Mothership Zeta starts when your PipBoy 3000 picks up a transmission coming from the alien star ship Theta crash site from the original Fallout 3. When you reach the crash site you're engulfed by a blue light and dragged upwards as you black out. When you wake up after getting probed (ouch!) you find yourself a prison cell with another captive whom you have to work with to escape the alien ship. Unfortunately there's not much of a story, you can recover alien captive audio logs but they don't really add an actual story to the Mothership Zeta quest line. The logs are merely window dressing.
Mothership Zeta is a very straightforward experience. Working with an interesting cast of other captives you have to escape the ridiculously evil grasp of the aliens before they dissect you, or worse. If there's one thing I liked about Mothership Zeta's plot it's the other human characters. There's a cowboy, an army medic from the Anchorage reclamation, a little girl from the time when the bombs dropped, a scavenger from the same era as your character, and a (unintentionally) hilarious Japanese samurai. Each character has a distinct personality which is a step above the rather uninteresting characters from the original game.
If I had to sum of the gameplay of Mothership Zeta in a brief sentence it would be "Operation: Anchorage in space"; by that I mean that it places its focus almost exclusively on combat and features barely any role playing elements. You essentially travel through corridor after corridor aboard the alien ship killing the aliens, their robots, and their escaping experiments while destroying the generators for key targets. I felt it got repetitive after the first thirty minutes, but there were some interesting environments such as the Waste Disposal area which was somewhat reminiscent of the trash compactor from Star Wars, complete with the opportunity for you to say "what an incredible smell you've discovered..."
Visually Zeta is pretty nice. The new textures are "clean" reflecting an enclosed, isolated environment, and the characters are all very detailed. Disappointingly outer space is clearly a cube with a texture pasted on it, you can actually see the edges of the cube when you look out the ship's few windows. From an audio standpoint Zeta is also pretty good, the voice actors for the new characters represent their characters well; I quite liked how the Samurai spoke actual Japanese, the developers didn't cop out and have him speak English with an Asian accent. The new alien weapons also have satisfying pew pew sound effects.
Mothership Zeta really isn't worth playing unless you really, really liked Operation: Anchorage. It places emphasis almost exclusively on action and has virtually no storyline. Still, the human characters were enjoyable but the other captives alone is not reason enough to pay for the two hour action-fest that is Mothership Zeta. I can't speak for everyone, but I felt that I had wasted my money since I got no genuine enjoyment out of the DLC.
+ Amusing cast of human characters
+ Some interesting areas
+ Above average voice acting for DLC
- Almost entirely action-based
- Little to no role playing elements
- Gets repetitive after the first thirty minutes
- No actual storyline
- Feels disjointed from the Fallout universe