Salvatore Leone is impressed with you on killing the Forelli Hitmen in the previous mission Freefall, who wanted to assasinate Salvatore. Now he wants to teach the Forelli's a lesson so he orders you to travel to Liberty City in order to kill all the Forelli's including the Don Marco Forelli.
CJ also requests to bring backup which is Kent Paul, Maccer, and Ken Rosenberg, however instead of going with you, they escape to Los Santos in a Sentinel, since Ken wantes to get out of the business and leave Las Venturas, and Carl tells salvatore that the three are dead. Now you are on your own, head over to the airport to where the Shamal is and fly to Liberty City.
Once you have arrived at Marco's Bistro, the restraunt, there will be a little Cutscene then heavy gun power will take place, try to take cover and kill all the Forelli's. Now the Don Marco will run to the back of the restraunt, follow him but remember that on the way, there will be more guards, kill them, and proceed to the backyard.
Now outside there will be three guards and Marco all shooting at you, kill the gurads while taking cover from the balcony, and as for Marco, the player can destroy his Sentinel parked outside and that will kill him, or you can just kill him with your gun.