On the eve of her marriage to the Marquis of Staines in 1909, Edwardian music hall star Harriet Green performs a farewell show at the Tivoli. During the after show party, Harriet's understudy Maudie becomes engaged to Lord Shropshire. Harriet is called away from the celebrations by an alarming message. Her former lover George Tredwell has heard about her forthcoming marriage and demands money in return for keeping Harriet's secret: she has an illegitimate daughter. Harriet shuns this blackmail attempt but decides that she must leave the country to avoid the scandal George threatens to uncover. She leaves their daughter, also called Harriet, in the care of her trusted dresser, Hawkie.
1934. Young Harriet attends an audition for a new stage musical directed by Leslie Benn, who performed with her mother at the Tivoli. Leslie sacks Tommy Thompson, a publicity man, after bad press about the show's leading lady. On leaving the theatre, Tommy bumps into Harriet, who has failed to make the chorus. They meet again later at Pryde's theatrical agency. Tommy and Harriet are alone in the waiting room when Maudie comes out of Pryde's office. She is shocked to see Harriet and realises that she must be the daughter of her old friend. Tommy persuades Harriet to appear on stage pretending to be her own mother. When Leslie sees Harriet perform 'A Little Springtime In Your Heart' dressed in Edwardian style, he agrees to the idea of promoting Harriet as the star who has never aged.
Maudie and Hawkie train the young Harriet in the mannerisms and performance style of her mother, and on Harriet's opening night the audience are impressed by the youthful figure and supple dancing of the star they remember from the Edwardian era. Harriet sings 'A Little Springtime in Your Heart', performs a Charleston, appears in a futuristic chorus number and dances a Polka. The show is a great success and journalists clamour to meet and speak with the youthful Harriet after the show. The Marquis of Staines suddenly arrives and Maudie, Tommy and Leslie fear that their deception will be revealed. The Marquis seems to be as duped as everyone else by Harriet's performance and assumes that publicity man Tommy is Harriet's son.
The father young Harriet has never known arrives and threatens to reveal the truth behind her grand deception. Tommy gives him £200 to keep him quiet. Tommy and Harriet begin to perform as 'Harriet Green & Son' but it becomes increasingly difficult to keep up the pretence, and after an argument Harriet decides to accept a marriage proposal from the Marquis of Staines. An iciness creeps into her relationship with Tommy, heightened by the fact that the Marquis sets 'mother and son' up in a plush house until the time comes for him and Harriet to marry. This compromising situation intensifies the young couples' feelings for each other, and as Harriet sings 'Dancing on the Ceiling' it is obvious that they have fallen in love.
On the opening night of Harriet's new show, the Marquis admits that he was aware of her deception all along. Tired of pretending to be much older than her years, Harriet goes on stage tearing off her costume and performing a frenetic tap dance. The audience begin to boo and hiss as she rips off her grey wig and they realise she is a fraud. Leslie and Tommy come on stage to beg the audience to give the young Harriet a chance to be a star, just like her mother, and Harriet is brought back on stage to tumultuous applause.
Despite the forgiving nature of the audience, Harriet still ends up in court, accused of defrauding the public. Singing along beautifully to a recording of her mother performing 'A Little Springtime in Your Heart', Harriet charms the judge and jury and is found innocent. Resuming her stage career, Harriet performs 'Over My Shoulder' and 'A Little Springtime in Your Heart' on a revolving stage with Tommy, Leslie and chorus girls. Tommy kisses Harriet's hand; she now wears a wedding ring.