The game is set in a 1979 New York where you are free to take on the role of one of The Warriors and roam the streets. You can initiate new members, steal car radios for cash, fight gangs and spray your Warriors tag over other gang's turf. The game starts out as a prequel to the movie where you can do what you like GTA style.

The game inevitably turns to the plot of the movie and the whole gang heads up to The Bronx for the big meeting. The story line then closely follows that of the film, bringing you into contact with gangs.
Thanks to Jimmy Clydesdale for bringing this to my knowledge.