The feud between the Capulets and the Montagues goes back far into the families pasts. Every generation passes it down. A fight starts in the square between the Capulets (Tybalt and servants) and the Montagues (Benvolio and servants). It gets out of hand and starts a riot. Prince Escalus has to break up the fight himself. He doesn't like the feuding and wants it to stop. He tells them that the next man who breaks the peace will be sentenced to death.
Later, Count Paris urges Lord Capulet to allow him to marry his daughter, Juliet. Lord Capulet tells Paris to wait two summers until Juliet is old enough. Juliet's mom asks her to think about marriage to Paris. Juliet hasn't really given marriage a lot of thought, afterall, she's only thirteen.
In town, Romeo is wandering around depressed because his girlfriend, Rosaline, dumped him because he only wanted her for one thing. His cousin, Benvolio, tries to cheer him up. Benvolio wants to takeRomeo to the annual Capulet party to find a better woman than Rosaline. Romeo says he'll go, but only because he heard Rosaline is going to be there. Romeo, Benvolio, and Romeo's best friend, Mercutio, go to the party wearing masks so that they won't be noticed. When Romeo first sees Juliet at the party, he falls madly in love. She feels the same way about him.
Tybalt, Juliet's cousin, recognizes Romeo and wants to kill him for crashing the party. Lord Capulet won't let him because he doesn't want a fight started during his party, and because he's heard that Romeo is a nice kid. Tybalt is furious and swears to get even.
At the end of the party, Romeo finds out that Juliet is a Capulet and she finds out that he is a Montague. Juliet says, "My only love sprung from my only hate, too early seen unknown and known too late." This basically means that she has fallen in love with her only enemy, Romeo. She had already fallen in love before she found out his name and now it is too late to change her heart or her mind.