When Benvolio and Mercutio were joking around, Benvolio sensed a fight is going to happen.
Tybalt came to challenge Romeo but since Romeo wasn't there he challenged Mercutio. Then they started fighting.
They were still fighting when Romeo came back from his wedding with Juliet tried to stop them.
When Romeo was blocking Tybalt went under Romeo's hand and stabbed Mercutio killing him. Then Tybalt runned away.
Romeo was furious at Tybalt that when Tybalt came back, Romeo started fighting him and then stabbed him which killed him. Benvolio told Romeo to leave the city which he did.
Then Prince Escaluse came with Lord Capulet, Lady Capulet, Lord Montugue, and Lady Montugue. Benvolio told the Prince what happened, but Lady Capulet didn't believe him and the Prine said that Romeo was exiled from this town.