It's your typical Quake2 story, you crash into a Strogg base and have to take it over, explained with some nice small cinematics to show where you are going to next between units. The first unit is a mix of outdoors, base and waste facilities. The indoor sections are probably the weakest of the game, but it soon gets better after this. From here it's more Quake2 themed levels, before finally reaching the moonbase. Unfortunately it didn't feel like much of a space unit, it uses all of the Q2 textures in a warehouse style. Doesn't compare to maps like COS3. Overall the build quality was just as good as Quake2, but there wasn't much originality as there were very few new textures.
Gameplay was excellent, with a few new soldiers that were great, except one who would lock onto as soon as you in his view. The Infantry were more powerful as they wouldn't take as long to stop and shoot at you. Most of the new enemies weren't that great - a beast that fires lasers and jumps at you, that was quite easy to kill. Also in the last couple levels the Gladiator was much tougher, sporting a new shield and new weaponry. Unfortunately he was no tougher than the normal Gladiators and you simply needed to use alot more ammunition before he dies.
The mission pack was never too hard, but you are always kept on your toes. Another complaint would have to be the easy ending with just one Makron to fight.
But overall a great mission pack and definitely worth getting.