After the overture, the curtain rises on the brightly lit ball at the Capulets' palace, where masked lords and ladies are singing and dancing. Tybalt congratulates Paris, who has not yet met Juliet, his bride-to-be. Capulet presents his daughter to the admiring guests and bids everyone be joyful on the occasion of her birthday. As the guests move into the other rooms, Romeo and Mercutio, with other young men of the Montague faction, arrive at the party in disguise. Mercutio would like to stir up trouble, but Romeo has had evil premonitions. Mercutio teases him with the “Ballad of Queen Mab,” the fantastic fairy of dreams. Suddenly Romeo catches sight of Juliet and is enchanted. When her nurse, Gertrude, hints of her forthcoming marriage to Paris, Juliet protests gaily that she wants to be free for a long while. Gregorio, a Capulet servant, calls Gertrude to help with supper, leaving Juliet alone so that Romeo, unmasked, is able to approach her. Together they sing a madrigal of antique and formal flavor which ends with the exchange of a kiss. Tybalt returns and Juliet identifies him as a cousin, revealing, to Romeo’s great distress, that she is a Capulet. Romeo has replaced his mask, but the furious Tybalt recognizes him nonetheless and moves to attack. Lord Capulet holds him back, unwilling to disrupt the party, and the Montagues retreat as the dance resumes.
In the shadows of the garden beneath Juliet’s window, Romeo recalls her beauty. Juliet appears on the balcony and, thinking herself alone, speaks aloud of her love for Romeo; he hears and they exchange vows of love. The appearance of Gertrude and Gregorio with a chorus of servants forces Romeo to retreat but, when they leave, the lovers continue their tryst and plan a secret wedding. Parting at last, they exchange a thousand farewells.
Romeo, then Juliet, comes to Friar Laurence’s cell to be married. After the ceremony, Gertrude joins in a quartet as all express their hope for the future in the praise of God.
In the street outside the Capulets' house, Romeo’s page, Stephano sings a serenade taunting the Capulets intending to provoke an incident. Gregorio engages him in a duel; Mercutio and Benvolio enter and become embroiled, as then do Tybalt and Paris. Romeo appears and tries to stop them all, but Tybalt is bent on revenge for the intrusion at the Capulets' party. Romeo refuses Tybalt’s challenge for, as he alone knows, he is now his kinsman. Mercutio takes up the duel in Romeo’s stead and is mortally wounded, whereupon Romeo avenges Mercutio by killing Tybalt. At last the Duke of Verona arrives, to chide the warring parties and punish Romeo with exile. He calls for peace, but Capulets and Montagues show their intent to defy him.
Juliet forgives Romeo for his part in Tybalt’s death, and they sing of the raptures of their wedding night. Hearing a bird’s song and knowing their separation imminent, they dispute alternately whether it is the lark heralding the morning or the nightingale whiling away the night. Romeo leaves just as Gertrude announces Capulet and Friar Laurence. Capulet insists Juliet is to marry Paris immediately. Alone with Juliet, Friar Laurence supplies her with a potion which will make her appear to be dead so she may escape the unlawful second marriage and Romeo may rescue her from the tomb. Alone, Juliet confronts her terrors and, placing the dagger in her bodice just in case, drinks the poison.
Romeo breaks into the tomb where Juliet lies and, delirious with grief, drinks a flask of poison. A moment later, Juliet begins to awake. Romeo is overjoyed and together they propose to escape, but it is too late. He recalls their parting at the song of the lark on their wedding night and collapses. She, finding the dagger, proclaims her joy at being able to die with him and preserve their love forever. Together they ask for God’s forgiveness and expire.