Travel to The Loop area of the map. The mission start point is available just outside a coffee shop.
Use the Profiler to find an access point.
Bring up the Profiler. The coffee shop is one one corner of a four-way intersection. Head diagonally across the street to the opposite corner and you'll find a building security router, along a wall. Hack it.
You have to play the hacking mini-game to get anywhere. When you do, you'll find yourself looking at T-Bone, who is at the facility he told you about. A scene follows.
Lead T-Bone to safety.
Unfortunately, T-Bone is not alone. You'll have to help him if he's going to get out of the area in one piece.
Return to Aiden and head up some steps through a walkway tunnel, as directed by the new yellow point on your map. Look at the pole above you and you should see a signal to hack. Do so and now you'll be looking down at the square where your friend hopes to escape.
You can see guards patrolling the area. Some of them likely have explosives on their person, so you can detonate those remotely from your camera, or do something similar to create a distraction. Then while they are dealing with that, you should run up and out of the tunnel. Launch a surprise attack and run and gun your way around the square, taking out the guards before they can comfortably find T-Bone. When you get the last of them, T-Bone will leave the scene while you linger.
Kill all fixers or escape.
Now you have to worry about your own escape. Quickly find a car in the vicinity, as you'll need it soon.
Defalt calls in the cavalry, and now you need to put them behind you. Staying around for a gun battle is too dangerous.
Drive along the street, but don't expect to rely on your usual antics. Defalt is hip to your little game, and he'll be hacking ahead of you. That means you should steer clear of steam vents and other such hazards, though it's possible you can catch the bridge as it starts to raise (handily leaving the fixers behind you).
The easiest way to get out of this situation is to drive by the water and find somewhere that boats are docked. If you can get to a boat, you can take out across the water and the fixers won't be able to follow. Then the mission will end.
NEXT: Act IV: The Defalt Condition