Aliens. If you don't believe in 'em, Bethesda sure wants you to. Mothership Zeta is the fifth and final DLC for Fallout 3. Depending on how you like to play the game and if you have Broken Steel or not, Zeta can be quick or it can be time consuming and painful.
The start is getting to the downed UFO in the north part of the map. You are then abducted and find yourself being interwoven into the lives of fellow abductees, ultimately defeating your abductors and making it back to Earth. This really doesn't vary off any paths previously laid down by the other DLC's, only in the fact that it's fairly more linear compared to others.
The gameplay is the exact same as the main game of Fallout 3 and other DLC's. The best part about this particular DLC is the fact that you are fighting in a totally new place: above apocalyptic Earth, and you get to use weaponry that bests basically anything on said Earth. The aliens look pretty realistic (besides the fact that they are shorter than my ten year old sister) and they pack a mean punch. The weaponry available is plasma based stuff, and it hits extremely hard. We're talking weapons that you get in about five minutes time in Zeta whereas an equal power weapon takes hours to get in the main game. There are not many new creatures to destroy, just three. You have your aliens, which come in several varieties including the vanilla fighter, the worker, and even a Captain. You also face the Abomination, which is the result of genetic splices that went awry, all taken from previously abducted things. These won't prove too tough, but when one pops around a corner and scares you, it seems like they are rather tough. You also face off with Alien robots, which aren't all that tough. The biggest threat are their cannons, and even these aren't that threatening if you get hold of an Alien Disintegrator early on.
The quest line this time by is generally linear, your biggest choice being which order you want to go in the three middle ways (all must be completed). Otherwise, your main objectives are to escape, and, while working with other abductees, destroy the alien threat and make sure that everyone on board is safe. The total time to do this shouldn't be too long, I did it in about four hours taking a break or two. There is an achievement for collecting the various logs scattered about the ship, and some of these are only obtainable once, and then they will be locked in a part of the ship you can't get back to.
You get only a handful of new environments to sample, most notably are the spaceship with it's various rooms and the spacewalk, which is a selling point in and of itself. The view is spectacular, and it really helps to give you a clear view of the devastation of the war that happened back on Earth. Overall, if you are a fan of Fallout 3, you'll want to pick this up for the weapons and experience which can help you get the level 20 and 30 achievements with extreme ease.