The Man in Chair, a mousy, agoraphobic Broadway fanatic, seeking to cure his "non-specific sadness", listens to a recording of the fictional 1928 musical comedy, The Drowsy Chaperone. As he listens to this rare recording, the characters appear in his dingy apartment, and it is transformed into an impressive Broadway set with seashell footlights, sparkling furniture, painted backdrops, and glitzy costumes. Man in Chair provides a running commentary throughout the show.
In the opening number, "Fancy Dress", the premise and characters of the show are introduced: it's the day of the wedding of oil tycoon Robert Martin and Broadway star Janet Van De Graaff, who plans to give up her career for married life. Those in attendance include aging hostess Mrs. Tottendale; her loyal employee known only as Underling; Robert's best man, George; Broadway producer Feldzieg, who is hoping to persuade Janet to forgo marriage and continue starring in Feldzieg's Follies; ditzy flapper Kitty, who hopes to take Janet's place in the Follies; two gangsters disguised as pastry chefs; self-proclaimed famed Latin lover Aldolpho; Janet's alcoholic Chaperone, who is supposed to keep her away from Robert until the wedding; and Trix, a female aviator.
The gangsters reveal to Feldzieg that their boss has invested in the Follies and wants to make sure the show is a financial success, which it presumably will not be without Janet. They tell Feldzieg that he must sabotage the wedding and make sure Janet stays in show business. Feldzieg enlists the vain, easily manipulated Aldolpho to seduce Janet and spoil her relationship with Robert. Meanwhile, in his room, Robert realizes that he is nervous about the wedding. To get rid of his "Cold Feets", he tap dances, and George, who is also nervous, joins in the dance. George notes that tap dancing could be injurious, so he suggests that Robert go roller skating in the garden instead, while wearing a blindfold to keep him from seeing Janet. Outside by the pool, Janet tells reporters that she is happy to be getting married and obstensibly doesn't want to be an actress anymore ("Show Off"), but her song evolves into a big production number.
In Janet's room, Janet is having doubts about whether Robert really loves her, and she asks the Chaperone for advice. The Chaperone responds with the extemporaneous "As We Stumble Along", a "rousing anthem to alcoholism", which, Man in Chair explains, the original actress playing the Chaperone insisted on including in the show. More helpfully, the chaperone tells Janet that she is feeling "drowsy" and must take a nap, giving Janet the opportunity to ask Robert if he loves her. Janet leaves for the garden, and Aldolpho enters, mistaking the Chaperone for Janet. The Chaperone happily pretends to be Janet and allows Aldolpho to "seduce" her ("I Am Aldolpho"). Janet meets the blindfolded and roller-skating Robert in the garden, and she pretends to be a French woman, "Mimi". She asks Robert how he met his bride, and he describes their lovestruck first meeting ("Accident Waiting to Happen"). Carried away by his emotions, Robert kisses "Mimi" because she seems just like Janet. Janet furiously storms off because Robert has "kissed a strange French girl".
Kitty, hoping to take Janet's place in the Follies, tries to demonstrate her mind-reading talents to Feldzieg, but he is unimpressed. The gangsters confront Feldzieg, threatening him with a murderous "Toledo Surprise" because he has not yet succeeded in cancelling the wedding. Feldzieg distracts them by insisting that they actually have singing and dancing talent, and they turn "Toledo Surprise" into an upbeat dance number. Aldolpho, with the Chaperone on his arm, announces that he has seduced the bride and the wedding is therefore cancelled, but Feldzieg angrily tells him he has seduced the wrong woman. Janet announces that she is cancelling the wedding, and Robert protests in vain that he only kissed "Mimi" because she reminded him of Janet.
Man in Chair announces that this is the end of the first act and the first record of the two-record set. He puts on another record, saying that the audience can listen to the opening of the second act of The Drowsy Chaperone, and leaves for the restroom. A scene set in an oriental palace appears onstage, with characters in stereotypical oriental costumes and the chaperone costumed as an Englishwoman in a hoopskirted dress ("Message from A Nightingale"). Man in Chair hurriedly stops the record, explaining to the audience that that was the wrong record—it was the musical The Enchanted Nightingale, not the second act of The Drowsy Chaperone. He finds the right record, and The Drowsy Chaperone continues.
In a musical dream sequence, Janet laments her lost romance and decides to return to the stage ("Bride's Lament"). Mrs. Tottendale tells Underling that the wedding will proceed as planned because "Love is Always Lovely" in the end. She reveals to Underling that she is in love with him. The Chaperone announces that there will be a wedding after all: she and Aldolpho are getting married. Mrs. Tottendale announces that she and Underling are getting married as well. Robert tells Janet that he loves her, and she admits that she was really the French girl and agrees to marry him. To appease the gangsters, Feldzieg tells them that he has discovered a new star: Kitty. He asks her to demonstrate her mind-reading talent, and when she "reads Feldzieg's mind", she announces that he is asking her to marry him.
George, now best man for all four weddings, realizes that he has failed at his most important task: finding a minister. Trix lands her plane in the garden, announcing she is about to depart for Rio. Because a captain on board a ship can perform marriages, everyone rationalizes that Trix, as a pilot, can perform marriages on board a plane, and she can fly them all to Rio for their honeymoons ("I Do, I Do in the Sky").
As the record is about to play the show's final chord, the power goes out in Man in Chair's apartment, and a Superintendent arrives to check the circuit breakers. The power returns, and the final chord plays. Alone in his apartment, a disappointed Man in Chair begins singing "As We Stumble Along" and is gradually joined by the cast of The Drowsy Chaperone, who cheer him ("As We Stumble Along (Reprise)").
- From Wikipedia