Brenda and Yvonne, two girls from the North of England, arrive at St Pancras railway station in London to seek fame and fortune. Their dream of glamorous city life quickly collides with reality when they realize they cannot pay for a meal at a greasy spoon café after Brenda has been robbed of her savings by a tramp.
Yvonne visits Carnaby Street in the hope of catching the eye of a trendy photographer, Tom Wabe, while Brenda stays behind to wash up. A messy scene ensues as ketchup is mistaken for washing-up liquid and everyone in the café is drenched in bright colours.
Yvonne excitedly tells Brenda that Wabe took her photo for a newspaper and paid her for it. However, when the paper drops that day, Brenda sees Yvonne mocked for being out of fashion, and decides to confront Wabe.
She finds him at a strange party served by robots (built and introduced by Clive Sword). While she tries to curse him out, Tom attempts to charm her.
The girls get a referral from their new flatmate for hostess jobs at a night club. On their first night there, Yvonne is summoned by affluent Bobby Mome-Rath, who gets her drunk and spirits her to an apartment he keeps for one-night stands. Brenda follows them, sneaks into the apartment, and tries to sabotage the tryst, adding laxative to Bobby's drink and ensuring that his bubble bath gets out of control. Meanwhile, Bobby's building supervisor, the Caretaker, spies through a hole in the ceiling. He falls through, giving Brenda and Yvonne a chance to escape. When they return to their flat, they learn the club has sacked them.
Brenda is hired by Charlotte Brillig, a dilettante heiress, to manage her exclusive design shop called Too Much, and is left in charge while her boss meets her ostensible main employee for lunch. When affluent hangers-on come into the shop, Brenda forces them to buy something. Although she sells a lot, Charlotte isn't pleased because nothing is left for rich customers to see. At the shop Brenda meets Tom, who asks her out to lunch. The restaurant, named Sweeny Todd's, is where Yvonne has found a job, serving pies in Nell Gwynne garb. A custard-pie fight starts at the party and spreads to the street. Yvonne is blamed for starting the pie fight and fired. In turn, when Brenda brings Yvonne to the shop, they learn Charlotte has capriciously shut the business down to go on an impulse trip to Greece, leaving both girls again out of work.
The girls watch a Candid Camera-style TV show on a television in a shop window entitled You Can't Help Laughing in which an old lady's house is demolished as a joke. When they walk back to their rooming house, they discover the program has also demolished their home as well. Yvonne, as the only name on the lease, wins a reimbursement cheque for £10,000. She decides to invest the prize money in becoming a pop star. Although the live recording of her single, "I'm So Young", is patently awful, it becomes highly polished after mixing, and Yvonne's out-of-tune voice is put in tune. It becomes a big hit and Yvonne becomes a star appearing on other programmes like Hi-Fi Court, a parody of Juke Box Jury.
Yvonne and Brenda drift apart. As Wabe's new girlfriend, Brenda goes to dinner on his canal barge home and stays the night. They spend the next day taking photos and she goes on to become a top model, while Yvonne's popularity wanes. Yvonne throws a plate at the TV when she sees Brenda in an advert for a new perfume called "Direct Action."
At a glamorous and star-studded party for Yvonne at the top of the Post Office Tower, Yvonne sits alone while everyone else enjoys themselves. Brenda watches the party on CCTV and sees Tom arrive to be mobbed by adoring girls. She gatecrashes the party only to see Yvonne humiliated when she falls in her own giant cake. Brenda finds the control to the revolving restaurant and turns it to full speed, ending the party in disarray.
The girls walk away in the early morning and decide to return home. The film ends with a reprise of the song Smashing Time.
