A temperamental opera diva arouses official suspicion that she is a spy, secretly gathering classified information to pass to enemy agents. A policeman who happens to be a talented amateur singer is sent undercover to join the opera company and try to find out whether there is any substance to the allegations. Once there, an immediate attraction springs up between the policeman and a female member of the company. But the diva also sets her sights on him and, used to getting what she wants, becomes the bitter rival-in-love of the other singer. The policeman lets his lady friend into his confidence, and the pair set about sleuthing. They finally prove that all the suspicions were justified and the diva is indeed a foreign agent.
In a nightclub, Teddy Wilson and his orchestra are laying down their last groove for the night. In the audience, Mr. Weathercoop (Russel Morrison) and his date (Virginia Pine) briefly discuss Wilson's band. Back in the kitchen, where Albert Ammons is hanging paper and Pete Johnson tunes a piano, a kitchen maid (Lena Horne) begins to sing about how she would like to perform in a new gown, rather than to wash dishes. Pete Johnson provides an impromptu accompaniment, and then Ammons joins in on a duet; a cutaway informs us that the Weathercoops are paying attention.
When his muse and girlfriend Nina (Tamara Desni) takes off with a continental lothario, composer and playwright Clinton Clay (Sinclair) is devastated and turns to drink for solace. His doctor (Sydney Howard) tries, with the help of Clinton's butler Neville (A. E. Matthews), to get him to pull himself together but all attempts fail as Clinton's behaviour becomes ever more unbalanced and every nurse they engage is sent on her way by him in quick order.
Farm workers harvest the crop whilst singing. At night they meet at the barn dance for more singing and dancing. Later, a man and a woman declare their love for each other, that "you could have knocked me over with a feather". The pair's song number is imitated by a male trio, one impersonating the woman.