Romeo (from the montague family) is lovesick for a girl named Rosaline. However, Rosaline is from the capulet family. And the montagues and the capulets are sworn enemies. And are even fighting right now.
Romeo and his friend Mercutio decide the gatecrash a capulet party to get over Rosaline. Although, this is where he sees Juliet (a capulet) and the two fall madly in love with each other.
The couple decide to get married in secret. But something goes wrong. Mercutio steps into a duel for Romeo and gets killed.
Romeo -incredibly furious- storms over to the capulet family and murders Tybalt. Because of this, he is banished from Verona.