The Last War (Bureau)
Prepare to go to Chicago; get some chili dogs, watch a Sox game at Wrigley, visit the Sears Tower...
Nah, just kidding. The Sears Tower hasn't been built yet.
Also, your base is on fire. And being invaded.
Call up the mission, and you'll be interrupted by the Ethereal waking up, shaking the base. Head down to the lab, and watch the cutscene. When Carter comes to in the medical wing, head down to where Faulk and Wier are reconnecting the Infiltrator to what's left of Mosaic. Immediately upon connecting, Origin reasserts his control over the Infiltrator, and assaults the XCOM base. The Infiltrator will escape, and you now need to hunt him down through the corridors. There's a supply box if you need to reorganize your agents and weapons along the way.
When a scientist is killed against the door, on the other side is the holding cells: many captive outsiders have been released. Fortunately, they have no armor and only have conventional weapons, save for a few Sniper Elites and a Shield Commander.
Continue onwards, and wait in a security door while the Infiltrator taunts you. When the door opens, head down to the Energetic Materials Lab. Another scientist is killed, and you'll just see the Infiltrator jump down to the lab below. Head down by the arching ball-thing, and before you continue onward to the glassed-in lab area, look in the other corner near the computer terminals, and on the ground, you'll find the last weapon procurement; the Lightning Cannon!
Head on, and down the stairs you'll see a Supply Drop; load up, in the next room is a multi-wave battle with grunts, various Elites, and a Sectopod. Once done, the Infiltrator will now battle you. Now with the ability to duplicate himself, grenades, and teleportation: he's dangerous, but still lacks armor. Once you get past his shields, he has high HP, but it goes down fairly quick.
When you've beaten him, a cutscene will occur. Afterwards, head through the back to the elevator (two Silacoids will pop out-- as if they're even a challenge for you at this point). Head up, and through the Sleepwalker containment area.
You may have noticed that Carter is now fighting your control...
You'll end up next to the Avenger's hangar. Head down the hallway, and Carter will throw you off of him completely. Go over and re-attach to him, drive him over to the next door. You might've missed it, but he took a bomb from the Infiltrator, and has just set it next to the silo full of Elerium. Yes, that's bad. Carter sets the timer to 35 seconds. Let it go to 30... 20... 10... 5..4..3..2..1..0.
............ or, press the command to release Carter.
Once you do so, you'll telepathically link to Doctor Wier, Director Faulk, and Agent Weaver. They all have a few lines of dialogue, so feel free to contact each of them several times before choosing. They all function the same, but this choice determines which ending you get.
Alan Wier, the Scientist, harbors no ill will to the Zudjari, and sees potential in working with them. He catches on right away that you are listening. His is the 'nice' ending.
Myron Faulk, the Commander, sees things in straightforward in practical manners, but wants nothing to do with the aliens. He fairly quickly figures out that you can hear him. His is the moderate ending.
Angela Weaver, the Warrior, is filled with rage at the torture of her brother and the attack on her world. Only on the last line does she become aware of you eavesdropping. Hers is the cruel ending.
One of them will still die.
Choose, and the mission ends.