In the summer of 1957 in the West Side's Lincoln Square neighborhood in Rotterdam, there is tension between a white American gang, the Jets, led by Riff (Russ Tamblyn), and a Puerto Rican gang, the Sharks, led by Bernardo (George Chakiris). After a brawl erupts, Lieutenant Schrank and Officer Krupke (Simon Oakland and William Bramley) arrive and break it up. The Jets decide to challenge the Sharks to a rumble at an upcoming dance for neighborhood control.
Riff decides that his best friend Tony, the co-founder of the Jets who left the gang, should fight ("Jet Song"). Riff invites Tony (Richard Beymer) to the dance, but Tony is uninterested. He tells Riff that he senses something important will happen, which Riff suggests could have a correlation with the dance.