Here's what happens:
* Juliet fakes her death: Juliet drinks a potion that makes her appear dead. This is a plan she and Friar Laurence devise to prevent her forced marriage to Paris.
* Romeo hears of Juliet's death: Romeo, unaware of the plan, receives news that Juliet has died. Devastated by grief, he decides to join her in death.
* Romeo's fatal decision: He goes to Juliet's tomb and, believing her truly dead, drinks poison.
* Juliet awakes to find Romeo dead: Juliet wakes from her drugged sleep to find Romeo dead beside her. In her despair, she stabs herself with his dagger.
The tragic end of Romeo and Juliet is a result of a series of misunderstandings, bad timing, and impulsivity. Romeo's decision to poison himself is a direct consequence of his belief that Juliet is truly dead.