Jerry Sandford, a successful touring entertainer, travels to London with his young daughter Peggy. After being spotted by producer Eve Barry whilst working the provincial theatres, he is moving to London to work in a new revue show called 'London Town'. After years of constant moving between rented flats, working in small theatres and dreaming of stardom, he hopes this job offer will be his big break.
On arrival in London, he quickly settles into a new apartment and befriends landlady Mrs Gates. He visits the producers to let them know he is ready for work, but no one is expecting him. Instead, he meets Patsy and Charlie de Haven, the show's stars. Charlie explains that he is the show's comedian and Jerry realises he has no part in the show after all.
He later meets Eve, the theatre's benefactor and the producer of 'London Town'. She offers Jerry a job as Charlie's understudy, telling him to watch Charlie and learn the act. Though disappointed, he is glad of the money and accepts. On the opening night, Jerry makes a point of wishing Charlie good luck but, suspicious of the threat he poses, Charlie is rude and obnoxious. Despite initial nerves, Charlie performs well, and at the after show party, complains to Eve that he doesn't like, or need, Jerry as his understudy. She insists on his staying.
'London Town' is a success and begins a healthy run. As the production moves into its second month, Charlie's performance is still going strong and he is increasingly rude to Jerry. His bad attitude extends to Belgrave, his long-suffering dresser who, like Patsy, begins to pity Jerry. By the third month, they increasingly object to Charlie and his mistreatment of everyone around him. When he is late one evening, they are pleased that Jerry finally gets a chance in front of a live audience. Surprised and unprepared, Jerry gets stage fright, forgetting his lines and almost fainting. Eve, however, has full confidence in him and tries to calm him down. They rehearse the routine, but Jerry still forgets his lines. A moment before he is due to go onstage, Charlie arrives; he's called Eve's bluff because he wanted to scare her into appreciating his input. Disappointed at his lack of nerve and angry at Charlie's manipulation of the situation, Jerry considers handing in his notice.
Peggy is upset that her father wants to leave the job. She learns the full story about Charlie's behaviour and, convinced that all Jerry needs is one night onstage, she hatches a plot to make Charlie miss a show. She buys a trick soap bar that leaves a bright green colour on whoever uses it, then enlists the help of Belgrave, asking him to swap the soap in Charlie's bedroom. Later, she persuades Jerry to go to the theatre to hand in his notice.
At the theatre, Charlie is characteristically rude to Belgrave who delights in tricking his boss into using the soap. Within seconds Charlie's face is bright green and he panics that he is ill. Jerry arrives in time to get ready for the show and, despite trying to hand in his resignation, goes onstage. By the end of the show, he has won over the audience and Eve employs him as Charlie's replacement. At home, Peggy is excited to hear Jerry's good news but begins to feel guilty at the underhand way that Charlie has lost his job.
Patsy begins to fall in love with Jerry who, with his newfound position in the show, can afford to buy an expensive apartment in the West End. Jerry's success adds to Peggy's guilt. She confesses her prank to her father, who immediately tells Eve. Explaining that Charlie had suspected he was the victim of a conspiracy, Eve tells Jerry that she needed Charlieout of 'London Town' and has offered him a new touring show. With everyone happy, Jerry invites Peggy and Mrs Gates to watch his star performance. At the end of the show, Patsy and Jerry reveal their love with an onstage kiss.
