Adapted from the Friml-Hammerstein-Harbach operetta, an opera singer tries to save her criminal brother and falls in love with the Mountie pursuing him.
Marie de Flor (Jeanette MacDonald) sings in an opera and then tells Teddy (David Niven) that she won't marry him. Marie learns her brother's parole was denied. She invites the Quebec Premier (Alan Mowbray) for dinner and sings "Pardon Me, Madame." Native Boniface (George Regas) tells Marie that her brother killed a Mountie and is wounded. Marie gets money from Myerson (Reginald Owen) and leaves.
The Mounties ride, and Sergeant Bruce (Nelson Eddy) sings "The Mounties." Bruce is sent to find John Flowers. Marie discovers that Boniface took her purse. At a saloon she sings "Dinah" for tips; but she is supplanted by the sexy singer Bella (Gilda Gray). Bruce follows Marie, who says she is Rose; but he knows she is Marie de Flor. She says she is meeting a tenor, and he helps her. In a canoe he sings "Rose Marie." They go to an Indian celebration. Bruce finds Boniface, but Marie says he's not her guide. Bruce asks the Indians about John Flower, while Marie blackmails Boniface into helping her. Marie says good-bye to Bruce. Below her window Bruce sings "Just For You." Bruce notes her name means flower and follows her.
Marie and Boniface try to swim their horses across a lake. Bruce rescues her, while Boniface rides away. Marie tells Bruce to leave her, but he camps nearby. She asks for food and runs from a deer. Bruce sings "Indian Love Call." He lets her sleep in his tent and searches her wet clothes. On the way to Hammond's Landing, Marie asks Bruce if he has to get his man. They sing the "Indian Love Call" together, and Marie tells Bruce that she loves him. Bruce leaves her at Hammond's Landing.
Marie finds John (James Stewart) and gives him money. He plans to leave, but Bruce comes in and arrests him. Bruce tells Marie that he knew John is her brother. Marie begs Bruce for John, but Bruce takes him. Marie sings the "Indian Love Call" and cries. At Montreal Marie sings in La Tosca, hears Bruce's voice in her mind, and faints. Myerson asks recovering Marie if they are taking proper care of her, and she declines a tour. Marie sings the "Indian Love Call," and Bruce answers her. They sing together and kiss.
From the sophisticated world of opera to the Canadian wilderness, music and love draw a singer and Mountie together even though he performs his duty in arresting her brother. In the process she is transformed from a complaining prima donna, who dislikes men, to a woman, who cannot sing without her man.