The plot twist got me scratching my head a bit as well. I figured we were trying to get rid of the evil scientist/dictator all the way through the game, only to find out I was the only one who knew this. Also, being the mute protagonist, I couldn't tell anyone else.
However, on the bright side, the game has three endings and all three are pretty cool, so all's forgiven on that front.
Other than that, Yahtzee doesn't give the game enough credit. It's by the numbers shooter with some fun gimmicks. The guns are satisfying and varried, enemies are introduced throughout the game at a steady pace, and the gimmicks/powers are also well paced. There's occasionally even a half-decent puzzle.
Basically, it won't blow your socks off, but as a rental or such it is well worth the money for a good Sci-Fi FPS. I had fun with it and I despise FPS games...