The assistant stage manager of a small-time theatrical company (Polly Browne) is forced to understudy for the leading lady (Rita) at a matinee performance at which an illustrious Hollywood director (Cecil B. DeThrill) is in the audience scouting for actors to be in his latest "all-talking, all-dancing, all-singing" extravaganza. Polly also happens to fall in love with the leading man (Tony) and imagines several fabulous fantasy sequences in which the director is free to exercise his capacity for over-the-top visuals in this charming 1920's era flick

In the late 1920s in a London suburb at the Theatre Royal, assistant stage manager Polly Browne cheerfully helps company members, including aging husband and wife Moyra and Percy Parkhill, who had previously performed in London's prestigious West End; former American child dancing phenomenon Tommy and handsome but distant star Tony Brockhurst prepare for the matinee performance of the musical The Boy Friend . Despite the sparse audience and the absence of the show's star, Rita Monroe, director Max Mandeville orders the show to begin. Upon learning that Rita broke her ankle when her high heel caught in a trolley car rail, Max frantically orders the startled Polly to go on in Rita's place. With encouragement from Tony and Max, a mortified Polly stumbles onto the stage and although knowing all of the lines and numbers by heart, freezes until prodded along by the others. Jealous chorus girl Maisie attempts to upstage Polly, who gradually warms up to the part of "Polly," a young girl pretending to be wealthy with a mysterious boy friend who will escort her to the evening's costume ball. Meanwhile, a chauffeur-driven sports car arrives at the theater, bearing famous Hollywood talking-picture director Mr. De Thrill, who is considering turning the show into a movie. Although delighted with De Thrill's presence, Max laments Rita's absence and the shoddiness of the troupe and theater, and wistfully imagines a Royal Command Performance of The Boy Friend with spectacular sets, costumes and a full orchestra. The company and stage hands continue helping Polly through her performance, pasting lines on props and dancing around her when necessary. Onstage, Moyra's character, Madame Dubonnet, suspects that Polly has invented her exotic boy friend to cover a lowly background. When Tony makes his stage entrance, the smitten Polly visualizes the Grecian party scene as a wild outdoor party and is nearly overcome. De Thrill's presence electrifies the company and Maisie sets about to impress the director at the expense of her dancing partner Tommy. Backstage, Rita arrives at last and offers the nervous Polly encouragement by telling her to "fake it." Despite Percy's complaints about working with Polly because she is a Cockney and his continual threats of walking out, he and Moyra nevertheless perform their numbers, to Max's relief. Onstage, Tony appears as a messenger delivering Polly's "Pierrette" clown costume for the ball. Admitting that she really does not have a boy friend to accompany her as a "Pierrotte," Polly impulsively asks the messenger to be her date to the ball and his assenting song, backed by music from a record player, prompts the watching De Thrill to envision a fantastic production number in silver, black and white featuring two giant turntables on which the entire troupe dances around Polly and Tony. During intermission, Polly is so captivated by Tony that she impulsively sings longingly to a picture of him while De Thrill's attempt to contact her is cut off by the enterprising Maisie. When Polly sees Tony and chorus girl Dulcie maneuvering to be alone together, she is heartbroken. [An intermission divides the story at this point.] The show resumes and onstage Polly lies to Tony the messenger, pretending to be wealthy, and the couple dream about moving to a simple home in Bloomsbury. After the messenger departs, Polly's friends demand to know details about her boyfriend. Backstage as Polly changes costumes, she wonders why Tony continually avoids her. Fed up with Maisie's continual over-the-top antics and upstaging, Tommy and the chorus boys teach her a lesson onstage by ruining their joint number. Soon however, no one in the company can resist the urge to play up to De Thrill and each number grows more and more expansive. As their onstage characters, Polly is reunited with Tony, the messenger, who reaffirms his feelings for her. When the wealthy Lord and Lady Brockhurst appear, Tony abruptly flees, prompting Polly's friends to suspect him of some criminal behavior. Max, who joins the show in one of the final numbers, is horrified when the chorus girl accompanying him enthusiastically strips off her nurse's uniform and performs a vamp number for the amused De Thrill. Finally at the ball, when Tony fails to appear, Percy and Moyra lead the troupe in a number pitying Polly, bereft in her "Pierrette" costume. Afterward, Tony, dressed as "Pierrotte," appears, startling the truly disconsolate Polly. In a moment that is not part of the show, Tony and Dulcie present Polly with a large cake on which Tony declares his love for Polly. The show then proceeds as Tony admits he is not a messenger but the wealthy son of Lord Brockhurst and apologizes for his background. Delighted, Polly admits that she is not rich and hopes Tony can still love her. As the show concludes with Tony and Polly vowing to live happily ever after, the troupe waits expectantly, hoping and imagining that De Thrill will select them to go to Hollywood. Polly is surprised to notice Rita leaving the audience in tears and only then realizes how good she has been as an understudy. Backstage, De Thrill disappoints everyone by revealing that he has decided to make Singin' in the Rain instead and leaves a card for Polly. Certain that De Thrill will take her with him, Maisie prepares to leave the company, but in the back alley when De Thrill sees Tommy do a unique dance step he recognizes him as his long lost son and the two are reunited, leaving Maisie behind. After the director leaves with Tommy, Polly reads his card inviting her to Hollywood, but tells Tony she would prefer for them to look for a home together in Bloomsbury.