Some Ancellotti chump named Charles "Chubby Charlie" Matteo plans to rat the Pegorinos out. He's going to fly a chopper out of Alderney into Algonquin to spill the beans. Niko and Phil have to shut him up. Or at least Niko does. Phil takes you to an old Sprunk Factory where the wise guys are hanging out. He leaves Niko to the job before taking your car and ditching you. So don't drive a car that you want to keep to this mission, because Phil just takes it.
Your given two options, have the first wave of guys see you by going in through the front or get the drop on them by sneaking in the back. Sneak through the back and catch the first two or three guys by surprise. It does not really matter as there are about twenty plus guys in here, but hey, every little bit helps. Make your way up the upper levels and chase Chubby Charlie. Don't bother aiming at him, because the makers of GTA 4 had a much more spectacular death plan for him, hence why they made him invincible until that point. Take out every other thug and chase Charlie up to the roof. Soon, Charlie while get into the chopper and it will take off. It will hover long enough for you to take it down. Pull out the most powerful weapon you own, and shoot it down.
A cutscene follows.
Charlie hangs on for dear life as he dangles from the helicopter. The chopper looses altitude, and crashes into the open roof of the factory. It crashes through all the levels until it hits the bottom, crushing and incinerating Charlie underneath it in an enormous inferno. It's fun to watch overkill isn't it?
Oh and by the way. If you want that sweet Super GT at the base of the building, steal it and park it at your safehouse as soon as possible because it disappears after the mission ends.