Capellio and Tebaldo address their followers advising rejection of an offer of peace to be brought by an envoy from Romeo, the man who had killed Capellio's son. Tebaldo states that he will avenge the killing to celebrate his marriage to Giulietta and he urges Capellio to hasten the moment when he may marry Giulietta and then avenge Capellio, who wants the marriage to take place immediately, brushing aside the objections of Lorenzo that Giulietta is ill with a fever. Tebaldo proclaims his love for Guilietta. Capellio's men urge him on and arrangements are made to have the wedding take place that day.
While the men proclaim their hatred of the Montagues, Romeo enters in the guise of a Montague envoy, offering peace to be guaranteed by the marriage of Romeo and Giulietta. He explains that Romeo regrets the death of Capellio's son, and offers to take his place as a second son for the old man. Capellio indicates that Tebaldo has already taken on that role and—together with all his men—rejects all idea of peace. Romeo accepts their challenge of war.
Giulietta enters proclaiming her frustration against all the wedding preparations, which she sees about her. Lorenzo enters, explaining that he has arranged for Romeo to come to her by a secret door and, when Romeo enters, he tries to persuade Giulietta to escape with him, but she resists in the name of duty, law, and honour, declaring that she would prefer to die of a broken heart. Romeo is distraught and the situation becomes more and more impossible for them both.
The sounds of wedding preparations are heard: she urges him to flee; he declares that he will stay as Giulietta continues to resist. Each leaves.
The Capuleti are celebrating the forthcoming marriage. All those assembled join in. Romeo enters in disguise and tells Lorenzo, who immediately recognizes him, that he is awaiting the support of his soldiers, one thousands of whom are assembled dressed as Ghibelines and who are intent on preventing the wedding. Lorenzo remonstrates with her, but suddenly, the armed attack by the Montecchi take place as they surge into the palace, Romeo with them. Giulietta is alone, lamenting the state of affairs. Then she sees Romeo, who has appeared, and again he urges her to run away with him. Capellio, Tebaldo and the Ghibelines discover them, and believe that Romeo is still the Montecchi envoy. As Giulietta tries to shield him from her father, he proudly tells them his true name. The Montagues enter to protect him and, in a concerted finale involving all from both factions, the lovers are separated by their family members.
Later, Giulietta awaits news of the fighting. Lorenzo enters and immediately tells her that Romeo lives, but she will soon be taken away to Tebaldo's castle. He offers a solution: that she must take a sleeping potion, which will make it appear that she has died. She will then be taken to her family's tomb where he will arrange for Romeo and himself to be present when she awakes.
With his followers, Capellio comes to order her to leave with Tebaldo at dawn. Her ladies beg her father to be kinder towards her. Proclaiming that she is close to death, she begs her father's forgiveness, but Capellio rejects her and orders her to her room. He then instructs his men to keep watch on Lorenzo of whom he is suspicious; they are ordered not to allow Lorenzo to have contact with anyone.
Romeo laments Lorenzo's apparent forgetfulness in failing to meet him as planned. He then hears the noise of someone entering. It is Tebaldo, and the two men begin an angry confrontation. As they are about to begin fighting, the sound of a funeral procession is heard. They stop as they realize that it is a procession for Giulietta. The rivals are united in remorse, asking each other for death as they continue to fight.
Along with his Montecchi followers, Romeo enters the tomb of the Capuleti. The followers mourn Giulietta's death. At her tomb and in order to bid her farewell, Romeo asks for it to be opened. He also asks that the Montecchi leave him alone with Giulietta. Realising his only course of action will be death, he swallows poison and, laying down beside her, he hears a sigh, then the sound of her voice. Giulietta wakes up to find that Romeo knew nothing of her simulated death and had been unaware of Lorenzo's plan. Urging him to leave with her, Giulietta gets up but Romeo states that he must remain there forever, explaining that he has already acted to end his life. The couple clings to each other. Then he dies and Giulietta, unable to live on without him, falls dead onto his body. The Capuleti and Montecchi rush in to discover the dead lovers.