Romeo is hiding out at Friar Laurence's, and Friar updates him on the Tybalt situation.
The Friar wants him to see the banishment as good news—yay! no executions!—but Romeo is too focused on the banishment part.
There's a knock at the door. It may be the Prince's men. Eek!
The Friar tells Romeo to hide, but Romeo refuses. Luckily for everyone, it's only the Nurse at the door. She and the Friar try to deal with Romeo, who keeps threatening really mature things like stabbing himself out of guilt for hurting Juliet.
The Friar comes up with a slightly plan that's better because it doesn't involve suicide: Romeo and Juliet can have one night together before Romeo leaves Verona.
Later, he promises, they'll be able to figure out a way to get Romeo pardoned by the Prince so he can come back to Verona and make his marriage to Juliet public knowledge.
Hearing this plan, Romeo recovers and runs off to see Juliet.
Quick Brain Snack: marriages in the Catholic Church (and lots of other churches) weren't consider valid unless they'd been consummated—i.e., the two people had to have sex. If Juliet and Romeo don't sleep together, Juliet's dad will be able to get the marriage declared invalid and marry her off to Paris.