In Shakespeare's Prologue to Romeo and Juliet serves as an exposition of sorts. In the form of a sonnet, the Prologue tells the audience that the play is set in Verona. We learn of the feud between the Montagues and the Capulets, and we learn that a "pair of star-cross'd lovers" come from these feuding families. Furthermore, the Prologue tells audiences that the only thing that can put a stop to tne feud between the two families is the death of the title characters.
While many question Shakespeare's motives for providing a summary of his play before Act 1 even begins (and indeed, he does give away the ending in his prologue), readers must remember that Shakespeare's plays were meant to be seen rather than read. The true genius of the play is evident as audiences--even those who know how the play will turn out--watch the tragedy with which the ending unfolds.