Party's Over is the penultimate storyline mission, given to you by Rocco.
Rocco will call Luis up saying that he's screwed and orders him to go to the public toilets in Middle Park, with Vincenzo alongside him. Following this the player must go to Maisonette 9, where a 'meeting' is taking place.
Rocco and Vincenzo have arrived there before you and are explaining to Gay Tony that the Ancelottis and Bulgarin want to see either Luis's or Tony's dead. Rocco will explain that Luis is on their side and want to choose Tony. And so he passes a pistol to Luis to 'shoot' Tony. After some hesitation Luis decides to gun down Vincenzo instead, with Rocco crying ut that they're 'dead'. The cutscene finishes and the player takes cover as Russians are about to enter. You must protect yourself and Tony - but Tony seems to hardly take any damage at all.
After about 4 waves of enemies Tony get a call from Dessie, pleading him to 'run while he can'. Luis now has to go outside as the Russians are arriving in sets of cars. A bike and a final car arrive when the first set is eliminated.
The mission ends with a cutscene that shows Luis pleading for Tony to come back, but he ignores this and walks away.