Outside a Manhattan tenement on a sweltering summer evening, some women pass the time ("Ain't It Awful, the Heat?"). The janitor takes out the garbage ("I Got a Marble and a Star"). The women gossip about one of the building's tenants, Anna Maurrant, and her extramarital affair with Steve Sankey, the milkman ("Get a Load of That"). They stop when she enters. Anna and young Sam Kaplan—who is in love with Anna's daughter, Rose—talk as Mr. Buchanan frets about his wife's impending childbirth ("When a Woman Has a Baby"). Anna's brutish husband, Frank, arrives and demands to know why Rose hasn't come home from work. After Frank goes inside, Anna pours out her frustrations and broken dreams, even as she continues to hope for a better life ("Somehow I Never Could Believe"). When Sankey walks by, Anna follows him, fueling the neighbors' gossip ("Get a Load of That" reprise). Lippo Fiorentino arrives with ice-cream cones for everyone, providing relief (comic and otherwise) from the heat ("Ice Cream Sextet"). Frank, not amused, rails against kids and modern society ("Let Things Be Like They Always Was"). The Hildebrand family enters. They are about to be evicted from their apartment because they can't pay the rent, even though oldest daughter has just won a scholarship ("Wrapped in a Ribbon and Tied in a Bow").
The tenants retire for the night. Sam stays outside to lament his isolation in the midst of so many neighbors ("Lonely Night"). After Sam goes in, Rose Maurrant finally enters, escorted by her lecherous boss, Harry Easter. Easter tries to seduce her with promises of a show business career ("Wouldn't You Like to Be on Broadway?"), but Rose rebuffs him ("What Good Would the Moon Be?"). Easter leaves as Frank enters. Mrs. Buchanan goes into labor, and Rose runs to summon the doctor. Mae Jones and her boyfriend, Dick, who have been out partying, do a jitterbug on the sidewalk ("Moon-Faced, Starry-Eyed"). When Rose returns, Mae's brother, Vincent, makes a pass at her. Sam comes out to defend her, and Vincent knocks him down. Rose comforts Sam, and the two share their dream of escaping the tenement's squalor ("Remember That I Care").
Early the next morning, Rose's younger brother, Willie, and the other children play on the sidewalk ("Catch Me If You Can"). Buchanan's wife has given birth. Rose tells Sam she is on her way to a funeral. Frank says he is going out of town, but he gets truculent when Anna asks when he'll be back. Rose tries to convince Frank to be kinder to Anna, but he rejects her advice ("There'll Be Trouble"). After Frank leaves, Anna sends Willie to school, telling him that he will make her proud some day ("A Boy Like You"). Rose tells Sam about Harry Easter's offer. Sam pleads with Rose to elope with him now ("We'll Go Away Together"). Rose leaves for the funeral, and city marshals arrive to evict the Hildebrands, as Sam remains seated on the stoop. Steve Sankey enters. Anna invites him up to her apartment. Suddenly Frank reappears. Sam tries to warn Anna, but Frank rushes upstairs and shoots Anna and Sankey. Frank escapes in the confusion as an ambulance, policemen, and crowds mob around. Rose returns from the funeral just in time to see her mortally wounded mother carried off on a stretcher ("The Woman Who Lived Up There").
Later that day, two nannies with baby carriages pass by and gossip about the murder ("Lullaby"). Rose returns from the hospital, where her mother has died. As Sam and his sister, Shirley, try to comfort Rose, more shots ring out: Frank has been captured by the police. Now remorseful, Frank awkwardly tries to explain to Rose why he committed the murders ("I Loved Her, Too") as the police lead him away. Sam once more declares his love and implores Rose to go away with him, but she has decided that she must go off on her own ("Don't Forget the Lilac Bush"). Two strangers enter, hoping to rent the Hildebrands' apartment. As evening approaches, the tenants again sit on the stoop, gossiping and complaining about the heat ("Ain't It Awful, the Heat?" reprise).
(Adapted from Mark N. Grant's synopsis at http://www.kwf.org/pages/ww-street-scene.html)