Of course, the reason this is applicable here is that MI is nonlinear, and there are several side trips and alternate paths available. I plan to follow those that contain important story information and bypass those that do not and/or interfere with the main (however much "main" can apply here) game progression, specifically Two For The Price of One and Robot World Arena.
The Prologue chapter screen is the first indication that MI's story is far more than what we're used to from M2. The screen is no longer a realistic image of a real event at all, indeed it's difficult to tell exactly what it depicts. Then the game begins, and we get our first taste of Infinity's atmosphere, overhauled graphics, and the work of the almighty Ydnar. Ne Cede Malis is dark, quiet, spooky, and very much inspired by Arrival (as admitted by its creator). The new Jjaro sound is very cool, and there's the mysterious ship out the window, later tentatively identified as Tycho's vessel. However, one slight disappointment is the terminals. The MI demo contained some very nice terminals, full of mysterious text and hints of greater mysteries to come. But now, it's just Durandal lamenting his fate. Note also that he doesn't even give us any instructions, there's no mention of the two switches to activate except in the pictures (which are unchanged).
As MI is the third part of a trilogy, its audience can be expected to have a great deal of experience with its gameplay, and so the mapmakers can really cut loose and make tons of interesting and difficult situations to deal with. The combat on this level is nevertheless reminiscient of Arrival as well, with the first major ambush of Fighters, and Compilers reading broken terminals jumping out of alcoves. This first area of the level features rooms containing a pattern buffer, an oxygen charger, and some ammo. Also in that room is a secret switch that will open both doors immediately; this is apparently a switch left over from the level's creation as a convenience. In the ammunition room there's a small shaft leading up into the cieling; we'll return to this later. Some other cool stuff in this area: A switch that opens a door halfway, just like the broken parts of Arrival. A small fan set into the wall that reveals a vent and a rush of air behind it when opened; could this be the famous Vent Core? :P A 1x health charger hidden behind a semi-secret switch in the first hallway. A door that lets you snipe (and get sniped by) a blue Fighter later on in the level.
There are two paths through the level to reach the switches, one (relatively) short and one long. We'll take the easy way first. Head up to the north end of the passageway and look for the hallway to the east with a Compiler in it. The terminal tells us a little about the new VacBobs, and then we reach a T-junction in the corridor. Heading west will eventually drop you into a small room that contains the first switch you must hit; the only way out of this room is into the corridors surrounding the 1x health charger, if you didn't open the first door to it you are now forced to start on the level's second path immediately.
Heading east from the T will bring you to a small chamber in the station's ventilation system and the other end of the Vent Core, which, after some jumping, leads us up to a Compiler reading a terminal. In the MI demo this terminal featured a rather nice poem; now it's been transformed into some sort of code. Hmm... One more thing about this new terminal: The "detailed" version of the Jjaro logo, which appears throughout the game, actually has a miniature map of Aye Mak Sicur superimposed on it. Some recognizable features are visible, such as the spur in the upper right and the large sewage room in the lower left. Beyond the terminal is a canister of napalm, which is probably a design oversight as this level has no flamethrower and we can't take it to the next level. Jumping down the hole beyond it will put you back in the ammo room at the start of the level.
To take the second path, go past the supply rooms and down through the yellow door. This puts you in the tunnels beneath the station, leading eventually to a complex area. Accessible from here is a room that gives a very strong sense of deja vu, a shotgun is hidden where you'd expect it to be ;) There's also a pattern buffer around here, and an elevator that will take you further up into the second path. After falling down a lighted shaft, you reach the second switch and a small cache of ammo, including some clips for the M-75 you don't have. Getting out of this path is somewhat harder than the first one; you'll have to brave some jumping puzzles and huge tanks of slime full of Compilers. If you're looking for a challenge, try to run through all the tanks and escape through the platform before the Compilers catch up to you; this feat has a margin of error of about 3 seconds, and if you fail you'll have lured them all into the same room with you. Good luck :P
Finally, you can exit through the double door at the start of the level and access a huge blue area that's seen some changes since the demo. The Hunters and VacBobs are missing, replaced with a bunch of Fighters. I believe some secret ammo was also removed from one of the high ledges. And the terminal has had its message of simple failure removed and replaced with some Thothy rambling about finding yourself. In hindsight this terminal explains what the Marine is going to try, but since we didn't know what to expect on the first run of MI it was just mysterious.
All in all, a very nice start to the game. Marathon Infinity took advantage of all the years of Marathon development to really take level design to a whole new level (heh). The maps are vastly more complex than those of Marathon 2, pushing the aging engine to the limit of what it can do - which turns out to be quite a lot. :)
