In 1920s Hollywood, with the film industry on the cusp of transitioning from silent movies to "talkies," Don Lockwood and Lina Lamont are a popular on-screen couple for Monumental Pictures. However, their off-screen relationship is purely studio-manufactured publicity. While Don is charming, talented, and well-liked, Lina has a grating voice and poor acting ability, which threaten to derail her career in talking pictures. At a party celebrating the success of their latest silent film, Don meets Kathy Selden, a young stage actress with whom he is instantly smitten, despite a rocky introduction where Kathy asserts her acting superiority over film actors.

The premiere of "The Jazz Singer" heralds the dawn of the sound era in cinema, putting immense pressure on Monumental Pictures to produce a talking movie. The studio hastily decides to convert Don and Lina's new silent film into a talkie, but it's a disaster due to Lina's voice and lack of vocal talent. During a screening test, the audience's laughter is relentless, and the studio executives are left scrambling for a solution. Cosmo Brown, Don's longtime best friend, and a musician at the studio, suggests they dub Lina's voice with Kathy's and turn the film into a musical. This solution seems perfect, as it would also give Kathy a chance to showcase her talent. However, tensions arise when Lina becomes jealous and spiteful toward Kathy, threatening the burgeoning romance between Kathy and Don, as well as the future of the movie itself.