The play begins with a street brawl between Montagues and Capulets. The Prince of Verona intervenes and declares that further breach of the peace will be punishable by death. Later, Count Paris talks to Lord Capulet about marrying his daughter, but Capulet is wary of the request because Juliet is only thirteen. Capulet asks Paris to wait another two years and invites him to attend a planned Capulet ball. Lady Capulet and Juliet's nurse try to persuade Juliet to accept Paris' courtship.
Meanwhile, at the house of Montague, Benvolio talks with his cousin Romeo, Lord Montague's son, about Romeo's recent depression. Benvolio discovers that it stems from unrequited love for a girl named Rosaline, one of Lord Capulet's nieces. Persuaded by Benvolio and Mercutio, Romeo attends the ball at the Capulet house in hopes of meeting Rosaline. However, Romeo instead meets and falls in love with Juliet. After the ball, in what is now called the "balcony scene", Romeo sneaks into the Capulet courtyard and overhears Juliet on her balcony vowing her love to him in spite of her family's hatred of the Montagues. Romeo makes himself known to her and they agree to be married. With the help of Friar Laurence, who hopes to reconcile the two families through their children's union, they are secretly married the next day.
Juliet's cousin Tybalt, incensed that Romeo had sneaked into the Capulet ball, challenges him to a duel. Romeo, now considering Tybalt his kinsman, refuses to fight. Mercutio is offended by Tybalt's insolence, as well as Romeo's "vile submission", and accepts the duel on Romeo's behalf. Mercutio is fatally wounded when Romeo attempts to break up the fight. Grief-stricken and racked with guilt, Romeo confronts and slays Tybalt.
Montague argues that Romeo has justly executed Tybalt for the murder of Mercutio. The Prince, now having lost a kinsman in the warring families' feud, exiles Romeo from Verona and declares that if Romeo returns, "that hour is his last". Romeo secretly spends the night in Juliet's chamber, where they consummate their marriage. Lord Capulet, misinterpreting Juliet's grief, agrees to marry her to Count Paris and threatens to disown her when she refuses to become Paris's "joyful bride". When she then pleads for the marriage to be delayed, her mother rejects her.
Juliet visits Friar Laurence for help, and he offers her a drug that will put her into a death-like coma for "two and forty hours". The Friar promises to send a messenger to inform Romeo of the plan, so that he can rejoin her when she awakens. On the night before the wedding, she takes the drug and, when discovered apparently dead, she is laid in the family crypt.
The messenger, however, does not reach Romeo and, instead, he learns of Juliet's apparent death from his servant Balthasar. Heartbroken, Romeo buys poison from an apothecary and goes to the Capulet crypt. He encounters Paris who has come to mourn Juliet privately. Believing Romeo to be a vandal, Paris confronts him and, in the ensuing battle, Romeo kills Paris. Still believing Juliet to be dead, he drinks the poison. Juliet then awakens and, finding Romeo dead, stabs herself with his dagger. The feuding families and the Prince meet at the tomb to find all three dead. Friar Laurence recounts the story of the two "star-cross'd lovers". The families are reconciled by their children's deaths and agree to end their violent feud. The play ends with the Prince's elegy for the lovers: "For never was a story of more woe / Than this of Juliet and her Romeo."