Singin' in the Rain starts off at the 1927 premiere of the new Monumental Pictures film The Royal Rascal. It stars Don Lockwood and Lina Lamont, two of the brightest silent film stars in Hollywood. Don tells the crowd all about his cultured, utterly refined upbringing, while flashbacks reveal that he's lying his pants off. He actually came up singing and dancing on the vaudeville circuit with his best friend, Cosmo Brown.
As Don and Lina soak up congratulations after the screening, it's revealed that they aren't as close as the fan magazines make them out to be. Also, Lina's a handful. She believes that she and Don really are a couple, even though they're not, and she has a voice so grating it could shred stainless steel.
On the way to the film's after-party at producer R.F. Simpson's palatial pad, Don and Cosmo get a flat. Don escapes a throng of rabid fans by jumping in the car of Kathy Selden. He hits on her; she tells him that she's a stage actress and thinks movie stars are undignified.
Needless to say, they do not hit it off.
At the party, R.F. shows a demonstration of talking pictures, the next big thing. Don spots Kathy as one of the dancing girls hired to entertain. She's mortified. He's intrigued. Don teases her, and Kathy attempts to throw a cake at him, except she misses, and hits Lina. Then Kathy runs off into the night.
Don spends weeks trying, and failing, to find Kathy. Meanwhile, The Jazz Singer, a talking picture produced by another studio, is a smash. R.F. decides Don and Lina's next movie, The Dueling Cavalier, has to be a talkie, even if Lina's got a terrible voice. Shortly thereafter, Don runs into Kathy on set; she's a chorus girl in another production. They both apologize and start falling head-over-heels for each other.
Lina and Don both take diction lessons to prepare for their first talkie. His go significantly better than hers. Filming during The Dueling Cavalier is plagued with problems, and the cut of the film that gets screened is a total disaster, littered with sound problems. Don, Kathy, and Cosmo regroup at Don's mansion and decide that they'll make The Dueling Cavalier a musical, and Kathy will record both Lina's speaking voice and singing voice.
Don and Cosmo pitch their idea to R.F. and he digs it. They rename the project The Dancing Cavalier and decide they'll make it a modern story, using all of the pre-existing costume drama stuff as flashbacks.
Lina finds out that Kathy's going to get on-screen credit as Lina's voice. She also finds out that Don intends to marry Kathy. Lina flips her wig and releases an interview that claims R.F. thinks she's a big star. R.F. confronts her, and Lina plays hardball. If R.F. doesn't remove Kathy's credit, she'll sue. Also, she wants Kathy to dub her voice forever, effectively ruining Kathy's showbiz career.
The Dancing Cavalier premieres, and the audience goes gaga for it—especially Lina's voice. The fans ask her to sing live, and Don, Cosmo, and R.F. are all for it. They put Kathy behind the curtain with a mic, and Lina in front to lip sync. Then the men raise the curtain, exposing Lina as a fraud. Kathy tries to run away, but Don stops her, and tells the whole crowd that Kathy's the real star.
The movie ends with Don and Kathy standing before a billboard for Monumental Pictures' latest production: Singin' in the Rain, starring Don Lockwood and Kathy Selden.