Mothership Zeta adds a different aspect to the Fallout series by introducing aliens into the mix
Simply put, a new radio signal is put out into the wasteland near the crash site of an alien space craft and the Lone Wanderer gets sucked up into a teleportation beam and captured aboard the title of the add-on, Mothership Zeta; a large craft inhabited by aliens that want to reclaim the earth as their own and seemingly study/destroy the human race in the process.
Now for the review of the game it's self;
There are a myriad of new weapons to be found about the ship such as the Alien Atomizer which is basically a space pistol, and the Alien Disintegrator, a space lazer rifle but with some extra kick and a cool reloading animation I think, plus several other weapons to play with.
Some new NPC's are found aboard the ship and lend a helping hand such as Somah, your cellmate the second you set foot onto the ship and a little girl named sally, plus a samurai, a cowboy and astronaut, all taken from various points in time to be used as test subject by the aliens.
The difficulty has been beefed up, but that may differ based upon your actual level in game but the aliens do provide a more than decent challenge, forcing you to use your most powerful weapons to take some of them down, what might have killed a regular raider may take a mini nuke to take out.
Some of the aliens do differ from one another but the game it's self doesn't distinguish by name, only by how many bullets it takes to finally kill it, the ones with the full suits and helmets are more difficult to kill than the ones without and it can get frustrating at times; I know I've died many times on something pretty simple such as taking out 4 or 5 of them in one room and dying relatively quickly, and I'm at the 30th lvl.
Their weapons are seemingly more powerful as well compared to what you may have, either regular earth weapons or even weapons you've picked up from the slain aliens themselves and they still do more damage than you, duck and cover and use stealth to your advantage to deal maximum damage to dispatch them quickly.
All and all I'd have to say that everything that's been added is welcome to me and provides a nice challenge for those that are either still playing through the main story line it's self or finished it and the other DLC content, it's a must buy for all Fallout 3 fans.
Oh and you get to battle a space ship with a death ray, awesome.