Read this before playing the Military Base Level:
(because it will save you a ton of grief)
This game is very straightforward and hard to get stuck, so I'm not going to write too much as it would probably take you MORE time to read a walkthrough than to just play... But the second Act in the Military Base Level called "Invisible Infiltration" can be extremely frustrating if you don't follow a specific series of events. I hope this video helps: