Lance Vance is back and together you fly to Prawn Island. You get a M60, so use it to kill as many enemies as possible on the mansion. If you kill all enemies in one section, Lance will fly to the next one. Be quick about it tho because if the health bar of the chopper reaches zero, you will fail.
Once Lance flew around the mansion, he will let you get out and you have to retrieve a briefcase from the roof. Shoot all enemies you can see outside the mansion, then enter the mansion carefully and shoot all enemies on sight.
Run up to the roof and get the brief case. Once you have it, Lance will pick you up and the mission is completed.
Rewards: $2,000, and ability to access Vice City Mainland (and associated missions.)