The Drowsy Chaperone JR. begins with Man In Chair addressing the audience. He discusses musicals, bringing up one of his favorites: The Drowsy Chaperone. He plays a record, immersing the audience in the show.

Mrs. Tottendale and her Underling enter, Tottendale admires her dress along with the rest of the cast, who appear onstage to introduce themselves ("Fancy Dress"). Everyone is gathered for the wedding of Robert Martin and Janet Van De Graaff, a "glamorous showgirl" giving up her career to be with the man she loves. Robert gives a speech about his soon-to-be bride, but George, the best man, speaks up, saying it is bad luck for the groom to see the bride before the wedding. George tasks Drowsy The Chaperone with keeping the couple apart.

The couple is whisked away, and the focus shifts to Janet's producer, Feldzieg, who is clearly troubled that Janet is leaving her career behind. His distress intensifies when a pair of Gangsters appear. The gangsters were sent by the largest investor in Feldzieg's company. They are tasked with ensuring that the wedding does not take place so the star of the show will remain. The gangsters threaten Feldzieg, who promises that the wedding will not occur.

Meanwhile, Robert is in his room with wedding-day jitters. He gives himself a pep talk that turns
into a lively dance ("Cold Feets - Part 1"). George enters, watching as Robert continues ("Cold Feets - Part 2"). When Robert finishes, George suggests that his friend roller skate to blow off steam. However, he also insists the groom wear a blindfold so he will not accidentally see Janet ("Wedding Bells #1 - Parts 1 & 2").

At that moment, Janet is being interviewed about giving up her career. Among the swarm of reporters is Feldzieg, who begs her to stay with his show. Janet refuses, insisting she does not want to show off while simultaneously putting on a performance for the crowd ("Show Off"). The Gangsters witness Janet's proclamation, telling Feldzieg it does not seem as if Janet wishes to stay in show business. Janet performs an encore ("Show Off Encore"). Feldzieg forms a new plan, recruiting unwitting Aldolpho by saying that the groom said he was a "scoundrel."
The producer suggests to an outraged Aldolpho that the man take revenge by kissing the bride. Aldolpho storms off in search of her.

In the bridal suite, Janet voices second thoughts about the wedding to her chaperone. She hopes the woman will give her some words of inspiration, but Drowsy simply expresses that in life, everyone is stumbling along, uncertain of themselves ("As We Stumble Along"). Still, Janet begs her chaperone for advice. Drowsy tells her to go to her fiancé. As Janet exits, Aldolpho enters, confusing Drowsy for the bride. Drowsy denies this at first, but when Aldolpho says that he must kiss the bride, the chaperone changes her story, telling Aldolpho that she is the bride. Before they share a kiss, Aldopho presses her to remember his name ("Aldolpho").

Meanwhile, Robert, still in roller skates and blindfolded, stumbles into the garden and nearly runs into a plant. Janet enters and cries out to him, but he does not recognize her voice. When Janet realizes this, she takes on the identity of Mimi, a woman from France. With her vocal disguise, Janet prompts Robert to talk about her. Robert relives the moment he realized he was in love, and Janet, caught up in the moment, shares a kiss with him. However, Janet quickly realizes her fiancé just kissed a woman that he did not know was her. She slaps him and rushes off, leaving Robert in distress.

Not knowing about the interaction in the garden, the gangsters approach Feldzieg in Tottendale's salon. They use menacing cooking phrases and movements to describe what they're going to do to Feldzieg, the final result being a "Toledo Surprise." However, Feldzieg is so caught up in their grace and rhythm that he asks them to repeat all this, turning the gangsters into surprisingly good performers ("Toledo Surprise - Part 1"). Aldolpho and Drowsy enter, Aldolpho proclaiming that he has kissed the bride and the wedding is cancelled. As soon as Feldzieg tells him that Drowsy is not the bride, Janet and Robert enter, again saying the wedding is off. Feldzieg joyfully joins the gangsters in another performance ("Toledo Surprise - Part 2"). The rest of the cast and a reluctant Janet and Robert join the dance ("Act One Finale").

After the cancellation of the wedding, Janet stands alone on her balcony, tormented by her broken heart ("Bride's Lament"). However, when Tottendale's underling asks if the wedding pews should be taken away, Tottendale says no, believing that love will prevail ("Love Is Always Lovely").

Drowsy enters the bridal suite and tells Janet that she is getting married to Aldolpho. Tottendale and her underling enter and announce that they, too, intend to be married. Robert bursts in next, insisting that he loves Janet and still wants to marry her. Feldzieg enters, arguing that Janet will miss the attention of show business if she follows through with the wedding. Janet turns to her chaperone for advice, and though Drowsy's advice is unclear to the Man In Chair and the audience, Janet agrees once again to marry Robert. As the gangsters menacingly approach Feldzieg, he finds a replacement for Janet and quite unintentionally a wife in actress Kitty. All of the couples proceed to say "I do" in one wedding, officiated by Trix ("I Do, I Do in the Sky - Part 1").

Suddenly, the power goes out in the Man's home, bringing the wedding joy to an abrupt halt. The Man's Superintendent appears, telling him he had been calling to warn him about this. The superintendent resets the breakers, restoring power, and the wedding scene suddenly bursts to life once more ("I Do, I Do in the Sky - Part 2"). The Man rushes the superintendent out, frustrated that the mood has been broken. The Man addresses the audience, acknowledging the musical's faults, yet sharing how much he still treasures it. He sings about it as the cast slowly trickles onstage, interacting with him for the first time ("Finale Ultimo and Bows").