At the far north side of The Loop area on the map, just south of the main river, you'll find the campaign point. Initiate the mission.
Track the auction briefcase and watch for hand-offs.
A man heads toward a briefcase and you need to watch its progress without being spotted. The man carrying the briefcase will talk to a newspaper vendor, so stand against the side of the stand, to the man's left, and wait.
Eventually, the man will leave the stand and head off toward a wall and make a swap with another man who is standing nearby, as Aiden comments on the situation on the phone to Clara.
You can watch the man walk toward the far side of the square. Then climb over the top of the newspaper stand (a great way to avoid being spotted by the contact who is watching from inside the stand), drop down over the other side, and run forward to get into a position behind a wall near the man as he continues his leisurely stroll.
The man will lean over a railing while you watch from your position nearby. Then he'll toss the briefcase over the railing while you stay in place. Wait where you are until your current objective completes as you receive a phone call from Clara and the man walks away from the railing where he dropped the briefcase into a waiting boat.
Tail the boat carrying the briefcase.
Now you can head after the boat, but you'll need to hurry.
There's a bike parked near where you should be hiding. Hop onto it, and follow the boat along the bank of the river. The route you can follow is fairly obvious if you pay attention to the map, and for the most part you'll be uninhibited by objects such as irritating pedestrians. However, it's possible to go too fast and that can create problems. As the bike comes up stairs and down another set, it can easily launch into the air if you're moving at too high a rate of speed, possibly knocking you off the bike or something similar.
Just travel at a moderate pace, watching the boat. Eventually, it will come to a marina and slowly weave through that, toward a building. Don't get ahead of the boat at that point. Just wait aroud until the boat finally comes to a halt, not far from the oversized chess set display along the shore. You'll receive your new objective then.
Locate the briefcase unseen.
Head toward the place where you last saw the boat, along the marina docks. As you get near to the point where the high-security area is marked in translucent red, jump off the right side of the dock and into the water. Swim toward the area and you'll gain another objective.
Infiltrate the marina to get the briefcase.
You have to pass security to find the briefcase, but it's not good being seen before you're ready. Swim forward, keeping to the dock, which should offer a shield from prying eyes.
As you get close, you can find a closed gate along the level of the water. Hack it to open it, and head inside and climb up a ladder to your right. From there, you can pass through a door to arrive on dry land, then make your way along that area, taking out the guards. There are plenty of distractions to make it easy. If you do get spotted, there's also plenty of cover.
Head around to the back side of the building and climb you'll find the briefcase. Aiden takes a moment to copy files once you find it, so hopefully you're not being chased and have eliminated all threats in the immediate vicinity.
Escape the marina.
Once you copy the contents of the briefcase, you just need to get out of there and the mission is done. Just run along the docks back to the mainland and you'll exit.
NEXT: Act II: Stare into the Abyss