Lawrence gets a sleeping drowth to seem like juliet is dead before the wedding. He sends juliet's plan to romeo. But because Mantua was having a hard time delivering the message to romeo, the news that was sent to him was that juliet is dead.
So, romeo bought some poison went back to Verona, and poisons Count Paris, and romeo himself drink the poison after. When juliet woke up and saw what happened, she stabs herself dead.