Don Lockwood is a very popular silent movie star who started out as a singer and dancer on the vaudeville circuit, then a stuntman, and then transitioned into becoming a star. The movie studio has created a fake romance between Don and his leading lady, Lina Lamont, to generate public interest about their films. The only problem with this is that Don cannot stand Lina, a vain, crass, and unlikeable woman who happens to look very good on film.
One night, when Lockwood is overwhelmed by a throng of over-enthusiastic fans who rip his jacket, he jumps into a passing car driven by a pretty young woman named Kathy Selden. She tells him that she is a stage actress and sneers at his film work, which infuriates, but also intrigues him. He runs into Kathy again at a party thrown by the head of his movie studio, R.F. Simpson. Simpson is excited to show a short segment of his new "talking" picture, but it doesn't seem to impress anyone. Don learns that Kathy, his new "stage actress" friend, is actually a chorus girl, and when Lockwood teases her for it, she throws a pie at him, misses, and hits Lina Lamont, who immediately has the studio fire Kathy. Later, Lockwood runs into Kathy working on another film, and the couple falls in love. They must keep their romance a secret from Lina, however, who would not approve of it.
When the very first talking picture, The Jazz Singer, is a huge hit with audiences, Simpson realizes he must convert the Lockwood and Lamont movie they are making into a talkie. The production of Simpson's first talkie is calamitous, mostly because of Lockwood's lack of credibility as a dramatic actor and Lina's horrible speaking voice/failures using a microphone. The test screening is a disaster and Lockwood's career is now hanging in the balance.
While lamenting the failure of the talkie at Lockwood's house, Don's best friend, Cosmo Brown, has an idea that might salvage the film. He suggests over-dubbing Lina's grating voice with Kathy's, and also persuades Simpson to change the film from The Dueling Cavalier to The Dancing Cavalier, a musical. The studio agrees and they set to work making a movie musical.
Lina is absolutely furious to learn that her voice is going to be dubbed by Kathy, and her fury intensifies when she discovers the romance between Kathy and Don. She tries to sabotage their relationship, and when she finds out that Kathy is going to receive a screen credit for her performance, she blackmails Simpson into withholding credit and engineers a power play to silence Kathy, a contract player and an unknown.
The premiere of the film is a tremendous success, but when the audience asks Lina to sing live, Simpson has to quickly improvise; Lina will lip-sync in front of the curtain while Kathy sings from behind the curtain, unseen by the audience. As Lina sings, however, Don, Cosmo and Simpson open the curtain behind her, revealing the deception to the audience. Lina is mortified and flees the stage. Kathy starts to run away too, but Don stops her and introduces her to the rapturous audience as "the real star of the film."
Don and Kathy perform a love song; the final shot of the movie shows Don and Kathy passionately kissing each other in front of a giant billboard advertising a film called Singin' in the Rain, starring Don Lockwood and Kathy Selden.
