At the house of Capulet in Verona. At a ball given by Juliette's father Capulet, her cousin Tybalt points her out to Pâris, whose bride she is to be. Mingling uninvited among the guests are Roméo, of the family of Montague, currently feuding with the Capulets, and some of his friends. Roméo has forebodings of disaster, but his friend Mercutio suggests mockingly that he has been visited by Mab, Queen of the Fairies, who presides over dreams. Teased about her forthcoming marriage by her nurse Gertrude, Juliette expresses her desire to enjoy her youth in freedom. Struck by her beauty Roméo addresses her. By the time Tybalt interrupts their conversation and they learn one another's identities, they have already fallen in love. Capulet, concerned for the success of his party, restrains Tybalt, who wishes to attack Roméo.
Hiding in the garden, Roméo overhears Juliette confess her love for him and her wish that he was not a Montague. He offers to disown his name if it will please her. His profession of his love is interrupted by Capulet servants led by Grégorio scouring the grounds for a suspected interloper - Roméo's page. They tease Gertrude with the suggestion that she has attracted a Montague follower. Roméo and Juliette agree to marry; he will make the arrangements and send her word. Gertrude calls Juliette to come inside.
Roméo tells Frère Laurent of his love for Juliette. She arrives and Frère Laurent marries them.
In the street outside the Capulet house, Roméo's page Stéphano sings a derisive serenade, warning the Capulets that their white dove is about to fly the nest. Capulet servants appear and a brawl develops, which is about to result in a duel between Grégorio and Stéphano. The row brings others to the scene, and Mercutio's remark that it is typical of the Capulets to pick on a child is overheard by Tybalt, who challenges him. When Roméo arrives, Tybalt wants to fight him, but Roméo, wishing to avoid a confrontation with Juliette's cousin, answers his insults calmly. Mercutio fights with Tybalt and is killed and Roméo casts prudence to the winds and challenges Tybalt, wounding him mortally. Tybalt dies in Capulet's arms as the rival families gather. A developing fight is prevented by the arrival of the Duke, who banishes Roméo.
Roméo and Juliette spend the night together. She has forgiven him for the death of Tybalt, as otherwise Tybalt would have killed him. She tries to delay his departure, only yielding when she realises the danger he is in. Accompanied by Frère Laurent, Capulet tells Juliette that she is to marry Pâris that very day, in accordance with Tybalt's last wish. At a signal from the friar, she remains silent and, left alone with him, she reveals that she would rather die than marry Pâris. He gives her a sleeping draught that will give her the appearance of death for a whole day, explaining that when she is taken to the family crypt, Roméo and he will be on hand to rescue her. She takes the potion.
Frère Jean is unable to deliver to Roméo the letter explaining what has happened. Believing Juliette to be dead, Roméo comes to the vault and takes poison. She wakes and they rejoice at their reunion, but Roméo begins to feel the effects of the poison. He tells Juliette what he has done and collapses at her feet. Finding no poison left, she stabs herself. As they die they pray for forgiveness.