The Conduit/Contagion

This is five days or five levels before the "Homeland" level. It's worth noting that you're working with Mr Adams whom he leads his organization called "The Trust" and wants to plot you to go against Prometheus and his alien force. Also this is the only level in which you get to try out your skills with the guns before you try out the ASE. So you won't be able to either scan secret messages and trust disks until the next level.

Agents and soldiers attack
You start out with the UPS or pistol and MP5 or a standard machine gun. You won't encounter aliens from the start. Instead you have to deal with enemy humans in suits and soldiers with rifles in an airport. Anyway, you'll hear the metal detectors running off like crazy so the best you can do is take out as many men in suits as you can before you move on past the metal detector but watch out for the person behind it.

Survive the terminal
You'll be thankful that the enemies you shot will drop off the health packs. You can't regenerate health at this point so make sure you get it before you move through the hallway blasting the enemies until you reach a big terminal room with all the chairs and the canisters. This part might be hard but you have to endure the pain you have to get through. So you have to clear the terminal not just blowing canisters near the enemies but also take out the snipers at the top, enemies coming out of left and right where the set of chairs are and soldiers coming down from a rope.

Down the escalator
Mr Adams will persist you to keep going after you clear the enemies, so you have to go through another hallway and on to an escalator. But the enemies detected you and they're coming up. Watch out on the left side where the canister is placed. After you clear the first group of enemies, quickly shoot the other canisters on the bottom of the escalator and take out the remaining enemy before you move to the next section which is the subway.

Blasting through the subway train
After you clear the enemies around the train on its side, go in and prepare to now deal with scientists with radiation suits and oxygen masks which they also carry guns. You have to go through three doors, shooting your way through the subway train until you reach a cutscene in which a freaked out scientist would blow himself up. In doing so, you survive the blast and discover the ASE which came out of the wreck. Mr Adams congratulates you and asks you to hang on to it for now but you'll be hanging on for the rest of the game.