Tensions exist between the Jets, a self-styled "American" street gang, and the Sharks, a group of young Puerto Ricans. The leader of the Sharks, Riff, swears to drive the Sharks, led by Bernardo, from the streets. Riff determines to challenge Bernardo that night at a dance at the gym, and prevails on his old friend and the co-founder of the Jets, Tony, to help him. Bernardo's sister Maria, newly arrived from Puerto Rico to marry his friend Chino, attends the dance and meets Tony, who at once falls in love with her.
Later, while the gangs begin to assemble at Doc's drugstore to choose a place and weapons for their rumble, Tony visits Maria on the fire escape of her apartment, and they pledge their love, promising to meet the next afternoon at the bridal shop where Maria works. At the drugstore, the Jets are nervous about the approaching meeting with the Sharks, but Ice advises them to play it cool, and when the Sharks arrive, an agreement is reached (at Tony's insistence) to have a fair, bare-handed fight between the two best fighters in each gang.
Next day, 
Tony
visits Maria at the shop and they enact a touching wedding ceremony. Maria makes
him promise to stop the fight between the gangs. As the rumble is about to begin,
Tony begs them to stop as he has promised Maria. Bernardo, enraged that Tony
has been making advances on his sister, pushes him back furiously. Switch-blades
appear and Riff is stabbed fatally by Bernardo; Tony kills him in turn.
In her room, Maria is preparing for her meeting with Tony. Chino bursts in and tells her that her brother has been killed by her lover, and rushes out to exact revenge on Tony. When he arrives, the lovers plan to escape to a place where they can be free of prejudice. The sorrowing Anita (Bernardo's girlfriend) knocks at Maria's door, and Tony leaves to hide at Doc's drugstore. Anita and Maria argue about Tony, but Maria persuades Anita to go warn Tony that Chino is looking for him with a gun. When she reaches the drugstore, Anita is abused by the Jets and, in a terrified rage, spits out that Chino has killed Maria in a jealous fury. Doc repeats Anita's message to Tony, and he wanders out into the streets calling for Chino to kill him too. He meets Maria, but their reunion is brief; Chino shoots Tony dead. Members of both gangs appear and, drawn together by the tragedy, carry Tony's body away.