Tony says that he will come find Maria after the rumble, and she asks him to meet her on the roof. "I'll come to your house," Tony says. Maria laughs that he could not meet her mother. He pulls a mannequin with a green dress out and pretends it's his mother, who loves to be in the kitchen. "She'll say, 'Skinny, but pretty,'" Tony says, confirming that his mother is plump. Maria tells Tony that her own mother is "delicate-boned," pointing out a different mannequin. She pulls up a male mannequin and pretends it's her father. "Papa might like you," she says, and Tony asks the mannequin for Maria's hand in marriage.
After getting the mannequins' blessings to marry, they arrange other mannequins to play Anita and Riff, the maid of honor and best man. Taking one another's hand, Tony and Maria process down an imaginary aisle and kneel, simulating a wedding. They put imaginary rings on each other's fingers and then sing a song of commitment ("One Hand, One Heart").
In an epic musical number, the Jets and Sharks prepare for the rumble, Anita prepares for her date with Bernardo, and Tony and Maria reprise their initial love song ("Tonight Quintet").
The two gangs arrive at a quiet, empty parking lot under a bridge. The Sharks climb over a fence and wait for the Jets, who arrive soon after. Bernardo is going to fight Ice, and Riff tries to initiate a handshake before the fight, but Bernardo refuses. They begin to fistfight, as Tony runs to the empty lot and tells them to stop. "Maybe he has found the guts to fight his own battles," Bernardo says, but Tony insists that they ought not to fight. When Tony extends his hand for a handshake, Bernardo bats it away and pushes Tony.
After Bernardo pushes Tony more and more, Tony gets ready to fight, but then decides not to. The fight escalates and Riff punches Bernardo in the face. The music escalates and Riff and Bernardo each pull out pocket knives. Even though Tony and the other Jets try to stop it, Riff insists on fighting with a knife. The other Jets hold Tony back as Bernardo and Riff fight. When Riff drops his knife, Action manages to get it back to him.
Riff goes to stab Bernardo, but Tony stops him. As Riff runs towards Bernardo, Bernardo stabs him. Enraged, Tony stabs Bernardo and a large fight breaks out among the boys. When they hear sirens, the boys scatter. Tony looks at Bernardo and stares at the knife he used to kill him, mortified, before yelling, "Maria!"
Anybodys runs into the lot and grabs Tony, telling him he has to leave, just as flashlights appear in the lot. Tony climbs over the fence and goes, leaving Riff and Bernardo's bodies there.
Meanwhile, Maria waits for Tony on the roof. She dances for a moment, when suddenly Chino comes to the roof and tells her that the rumble took place. As he stutters through telling her what happened, Maria begs Chino to tell her what happened to Tony, which only angers Chino. Chino yells, "He killed your brother!" and flees from the roof.
Maria runs after Chino, accusing him of lying, as a child downstairs tells their parent that Bernardo is dead. Maria runs to her room and prays to Santa Maria that it isn't true. Suddenly, in the mirror, she sees Tony come into her bedroom. She runs towards him and punches his chest, screaming and calling him a killer. When she collapses in his arms, Tony tries to comfort her and tells her that he didn't mean to, and that Bernardo didn't mean to kill Riff either.
Tony asks for Maria's forgiveness and tells her he is going to the police, but she begs him to stay with her. They sing a duet about going to a place where they will be safe and able to love each other exactly as they want ("Somewhere (There's a Place for Us)").
A police car drives through the neighborhood, as A-Rab runs down an alleyway and finds Baby John crying on a roof. Baby John is embarrassed to be crying, but A-Rab sits with him for a minute. "I wish it was yesterday," A-Rab says, and he leads Baby John to meet up with the other Jets, who are hanging out in an alley.
The Jets discuss the fact that Tony really came through for the Jets, and Graziella cries about the fact that Riff is dead. The boys begin to fight about whether or not they should fight the Sharks, with Action suggesting that they ought to finish the Sharks off. As a fight breaks out between them, a man throws a bottle down into the alley from an apartment above, and the Jets go into a garage. Ice, who has taken over as leader of the Jets, urges everyone to stay cool, and sings a song about it. Everyone joins and dances ("Cool").
In the dress shop, Maria and Tony create a fantasy of their wedding, a utopian vision in which everything goes smoothly and their racial differences and gang rivalry do not matter. The mannequins who stand in for the people in their lives, their parents and their friends, are very supportive of the union, and Maria even dons a veil for the fake nuptials. The simulation is at once sweet and sad, as both Maria and Tony seem to know that it is a fantasy, a complete fiction. Only in a dream or a simulation can Tony and Maria dream of a union; there are too many impediments to their love otherwise.
The music, written by the acclaimed composer and conductor Leonard Bernstein, is as complex as it is compelling. Particularly in the "Tonight Quintet," Bernstein's handle on the drama of the musical, as well as the various characters, is virtuosic. The song weaves new melodies into the familiar "Tonight" that Tony and Maria sang earlier. In this way, we see that the tension between the two gangs is building to a (literal musical) crescendo even as the two lovers are protected by their bubble of mutual affection.
Tragedy strikes at the rumble, when Bernardo kills Riff and Tony kills Bernardo. Tony arrives at the rumble eager to stop it, but he soon finds that the impetus to fight, the tension between the two gangs, is too strong to stop. While he does his best to stop the violence, when he sees Bernardo kill Riff, he is filled with rage and kills Bernardo in retaliation. The violence just begets more violence, as the characters are so overwhelmed by the competitive energy that has led them to the rumble. The minute anyone is killed, however, the color drains from the boys' faces and we can see just how lost and young they truly are.
In an instant, the beautiful love shared between Tony and Maria gets much more complicated. While Tony was supposed to diffuse the tension at the rumble and create a peace treaty between the rival gangs, he ends up unable to change the course of the violence, and in a fit of passion, ends up killing Bernardo to avenge the murder of Riff. From one scene to the next, Maria and Tony's love changes from pure and suspended in a blissful fantasy of peace to marred by tragedy and betrayal. This irrevocable tragedy is on display in Tony's behavior following his murder of Bernardo. He raises his arms in despair and calls Maria's name, barely able to run from the cops who are showing up at the scene.
While the murders of Riff and Bernardo seem like climactic moments in the plot, they are in fact just incidents that lead towards the end of the film. The Jets and the Sharks continue on, and even Tony and Maria continue to build their inseparable bond, in spite of Tony being Bernardo's killer. Without the respective leaders of the two gangs, those left behind are left to piece together a new organization, and Ice, the Jet who takes Riff's place, urges everyone to keep it "cool" in the wake of the tragedy.