In act one Romeo meets Juliet at a party, held by the Capulets. When he sees her he instantly forgets about Rosaline, a girl who rejected him, breaking his heart. They instantly fall in love with each other "Romeo: I ne'er saw true beauty till this night"(1.5.29). They make a plan to get married and be together forever.
In act two, with the help of Friar Lawrence, Romeo and Juliet get married "Friar: Till Holy Church incorporate two in one."(2.6.59). Romeo and Juliet do this without their parents approval. The are also family rivals so they must keep their love secret.
In act three Romeo killed Tybalt, Juliet's cousin, because Tybalt killed Mercutio. The prince banishes Romeo, so he and Juliet can never be together. Juliet is sad and starts to get depressed.
In act four Friar Lawrence hatches a plan to help Romeo and Juliet be together. He gives her a potion that will have her sleep for 42 hours(4.192-94). Her family will bury her and Romeo would come and dig her up. Juliet takes the potion and the plan works well, at least so far.
In acts five Romeo did not understand the plan and he thinks Juliet is actually dead. So he goes to her tomb and drinks poison to be with her forever(5.3.114).
Later in act five Juliet wakes up from the potion and finds Romeo dead so she kills herself with his knife to be with him forever(5.3.115). Then Friar Lawrence finds them dead and he reports it to the prince and watchmen.