What happens
In Mantua, Romeo wakes strangely cheerful from a dream in which Juliet found him dead and kissed him back to life. Then his servant Balthasar rides in with real news: Juliet is dead and buried in the Capulet tomb. Romeo does not pause. "Then I defy you, stars," he says, and resolves to lie beside her in the vault that very night. No letter has come from the Friar. He goes straight to a poor apothecary — a starving man who breaks his own city's law against selling poison because Romeo's gold matters more to him than his life — and buys a dram strong enough to kill at once. Romeo sets out for Verona to die.
Why it matters
Everything turns on what is missing here: the Friar's letter. Romeo acts on the one piece of news that is not true, with total speed and total conviction. "I defy you, stars" is him seizing control of his own fate — and it is the choice that fulfills the prologue's prophecy. The play's cruelest joke is that his decisiveness is what kills him.
The dream is a small, devastating irony. Romeo dreamed Juliet's kiss brought him back to life; the reality will be the reverse — he dies, and her kiss comes too late. Shakespeare hands him a glimpse of a happy ending moments before taking it away.
The apothecary scene is a quiet flash of social anger. A man sells death because he is starving, and Romeo notes that gold is the real poison, doing more harm in the world than the drug he is buying. Even here, hurtling toward his own death, the play pauses on who is desperate, and why.