In The Begining:

Niko arrives to America to find a man who he thought was the reason for an ambush that killed all his friends when he was in the army. When he arrives he finds his cousin Roman who doesn't exactly have the lavish lush life that he says about in all of his letters.

Incidental Accident:

When Niko and Roman piss off the wrong person they burn down not only the appartment they were living in, but also the cab company that Roman runs as his buisness. When the whole thing goes up in flames they have to move to a new part of town and start over, while this has very little concequence of the end of the story it does move you through the game into the bigger areas of the game.

Middle:

The world is your oyster once you get your cousin out of debt. The majority of the game is meeting new people and climbing up the social ranks of the Liberty City underbelly. As you go through this section of the game you are still searching for this man you came to America for, but in order to find anyone with any information or connections you have to start at the bottom and work your way up. The thing you do the most is shoot people, generally it is because there is money to be had or drugs to be found, or some other nefarious job that is poorly influencing the young teenage boys who play this game.

Climax:

Eventually when you get hired by some government official, who turns out to be crooked as a dogs hind leg, and he makes you work for no pay. Only the threat that he will throw the book at you if you don't kill who he tells you, when he tells you. This guy eventually gets you the location of the person you are looking for. When you track this guy down you are presented with an option, let him live or shoot him in the head. Depending on what you do is how the game ends.

Ending:

The ending is fairly simple, someone dies. The choice comes when you decide to kill your rival or not. If you kill him, thugs come later in the game and kill your cousin as revenge. If you don't kill him then the McReary sister dies. After all this murder is said and done, you are free to continue playing the game to get the fabled 100% completion stat. This means you have finished every optional mission, found ever easter egg and such. This takes way to much time, and isn't exactly worth it but its good to note when the story is finished you still get to play in the world.