in scene one there was an incident between the Jets and Shark leader, Bernardo, escalates into an all out fight between the two gangs and someone stabs Bernardo in the era and when the cops came the police try to stop the fight in between them and the cops told the sharks to leave and they started to sing and the jets and the cop started to make fun of them. 
At the dance in the local gym, the group is divided: Jets and their girls on one side and Sharks and their girls on the othe. Riff and his lieutenants move to challenge Bernardo and his lieutenants, but they are interrupted by Glad Hand, the chaperone who is overseeing the dance, and Officer Krupke.
Cut to the dress shop where Maria and Anita are discussing love and hemlines. She is so not going to let Maria wear that plunging neckline. Maria knows that her brother Bernardo wants her to be with Chino, but she's just not that into him.