You will begin this mission in the backseat of a car being driven in an unusual russian fashion towards a man they believe to have a nuclear weapon.
I highly recommend losing the silenced starter-weapon as fast as possible and using your secondary primary (2) since it is utter garbage in both recoil, accuracy and damage.
Leave the car and run into the parking lot.
Enter the office to your left and locate this device and cut the cables off.
After going down one level, you'll encounter more enemies who haven't noticed you yet. I recommend taking the upper platform and seeking cover in the door-way while also shooting the carlights blinding you.
Make sure to proceed VERY slowly with this mission. Scripted massive explosions and buggy insta-deaths are everywhere, even with an up-to-date campaign. In other words, stay back, fire and let your buddies go first.
You'll continue further into the building, running into even more enemies. I recommend throwing a grenade or two here due to the proximity of enemies and the amount of cars that can explode along with the annoying lights.
And remember; Take it slow or you're going to blow up.
After a long corridor sequence, you'll need to breach a door housing a large room with several enemies inside. Enter and clear as fast as possible and then quickly put your gasmask on or you will die.
After going up a pair of stairs, there will be another quick-time event. Tip that fool down over the guard railing and proceed forward.
You'll run into some rooms resembling much the graphical layout of Mirror's Edge, going from not a soul in sight.
To large concentrations of enemies with little to no cover at your end. Fire away.
Refill ammo if needed.
Rather long combat-sequence ahead, but at close proximity. Shotgun is golden here.
Cover is almost non-existant here as mentioned earlier so keep your weapons loaded.
Breach another door and do NOT shoot the guy on your HUD. It will shoot the nuke and leak out radiation, ending the mission.
Continue to chase after him, cleaning enemies as you go, no rush just yet.
Another quick-time event close to the elevators.
Finally, you get to the building exit where an army of police-men are having a shoot-out. You are prompted to stay back and then later only deal with the surviving scrap of enemies. Gasmasks off.
Deal with the remaining police-men and run after the guy with the nuke.
One of your teammates will get hit by shrapnel and die. At this point, you can just chase after the guy non-stop.
Be ware of the quick-time event up ahead.
If you fail here, the mission will end. If not, the mission will be completed. Buttons included are M1, E and space-bar.