The film takes place in Verona, Italy, where a violent, ancient feud between the Montague and Capulet families ravages the city and terrorizes the townsfolk. One day, a brief spat between the enemy families erupts into a full-fledged street brawl, which is then broken up by Prince Escalus, who warns that from now on, those who fight will be put to death.
Romeo Montague, a handsome youth of about 17, is miserably in love with a girl named Rosaline (who makes no appearance in either the film or the original play). Meanwhile, Juliet Capulet and her family are preparing for a great masquerade ball at their home that night. A wealthy, older bachelor, Count Paris, has asked the permission of Juliet's father, Lord Capulet, to marry his only daughter, which her father asks to postpone until her approaching 14th birthday. That night, Romeo and the other Montagues secretly attend the Capulet feast. The objective of the evening is for Romeo to overcome his love for Rosaline, and when he meets and dances with Juliet, the two teenagers immediately fall in love.
Later, in one of the most famous scenes in all of Shakespeare's works, Romeo approaches Juliet's private garden and hears her speak of her feelings for him from below her balcony. He accidentally startles her, telling her that he has the same feelings and that he would renounce his family name if it meant that he could be with her. The two make plans to see each other again the next day.
Romeo goes to Friar Lawrence, a friend and confidant of his, and asks him to wed the two lovers in secret. The Friar is frustrated that Romeo's passions have switched from Rosaline to Juliet so seamlessly, but agrees to marry them, citing his hope that it will bring peace to their families. Next, Juliet sends her Nurse to meet Romeo in public. Romeo asks the Nurse to have Juliet meet him that afternoon to get married in secret. The Nurse is elated and agrees. At the chapel, the Friar warns Romeo to love Juliet in moderation, lest their love meet its untimely end. He then proceeds to marry them.
The next day, another battle in the street breaks out between one of Romeo's best friends, Mercutio, and Juliet's cousin, Tybalt. Tybalt initially wants to confront Romeo, citing his intrusion at the masquerade ball as a grave insult. Romeo, however, is riding the high of his new marriage to Juliet and greets Tybalt with warmth and respect. Thinking he's mocking him, Tybalt insults Romeo, and Mercutio jumps to defend his friend. He and Tybalt duel, and though Romeo tries to stop the fight and create peace, remembering the prince's warning, he accidentally gets in between them, and Tybalt mistakenly stabs Mercutio under Romeo's arm.
Mercutio dies a horrific, drawn-out death in which he begs for help and his friends only laugh, thinking he's kidding. When they finally realize that he was concealing a mortal wound, it is too late. Romeo becomes enraged and avenges the loss of his best friend by dueling with and slaughtering Tybalt. The bodies of Mercutio and Tybalt are brought before the prince, where Benvolio, another one of Romeo's best friends, explains to him that Romeo acted to avenge Mercutio. The prince, a relative of Mercutio's, shows mercy and banishes Romeo from Verona instead of sentencing him to death.
Unaware that Juliet has already married Romeo, Lord Capulet arranges for her to marry Paris. Juliet refuses, sending her father into a rage in which he threatens to throw her out on the streets. The Nurse implores Juliet to forget Romeo and marry Paris, which Juliet sees as a monstrous betrayal. She seeks out Friar Lawrence and together they concoct a plan to fake Juliet's death so that she can escape and be with Romeo. The Friar gives Juliet a potion to drink that will make her look dead for around 42 hours. The plan is that the Capulets, thinking her to be deceased, will send Juliet to her tomb, and then Friar Lawrence will send another Friar, Friar John, to alert Romeo of his lover's fake death. Meanwhile, Friar Lawrence will plan to meet Juliet at her tomb and help her flee.
Friar John is delayed in delivering Romeo the letter, however, and a friend of Romeo's named Balthasar witnesses Juliet's funeral and reports to Romeo that the false news she has died. In despair, Romeo goes to Juliet's tomb and, bereft at what he thinks is his lover's demise, kills himself by drinking a poison that he bought in Mantua. Friar Lawrence arrives and finds Romeo's body besides Juliet's. She then wakes up, and the Friar tries desperately to get her out of the tomb without noticing Romeo's corpse. She sees it, however, and refuses to leave him. Hearing the police approaching, the Friar flees. Juliet tries to find a way to get the potion out of Romeo, and when she can't, she uses his dagger to stab herself. She collapses onto his chest, dead.
After the police arrive, the two families and the Friar attend the funeral for Romeo and Juliet and the families agree to end their feud.