A huge crowd gathers in front of Hollywood's Chinese Theatre, hoping to catch a glimpse of the stars arriving for the glittering premiere of the latest Lockwood and Lamont picture, "The Royal Rascal." The dashing Don Lockwood and the lovely Lina Lamont - who have been linked romantically in the press - are questioned by gossip columnist Doris Bailey, who persuades Don to share the story of his rise to stardom. As Don obliges with a tale of training in exclusive academies and performing in splendid symphony halls, a flashback reveals the less-than-glamorous truth: Don and his friend Cosmo Brown started out tap-dancing on the streets, graduated to performing vaudeville in seedy halls and ended up playing "mood music" for silent films (Fit as a Fiddle). Through sheer luck, Don got work as a stunt man, where he attracted the attention of producer R.F. Simpson, who cast Don in his first leading role opposite Lina Lamont. She ignored him when he was a lowly stunt man but pursues him now that he is a star.
"The Royal Rascal," the latest Lockwood-Lamont silent film, is a big hit, and Don is sure to do all the talking when he and Lina accept congratulations. We can see why after the crowds disperse: Lina's voice is a nasal, shrill screech: the vocal equivalent of fingernails scraping across a blackboard. Lina, a sparkly but low-watt bulb, does not see this, nor will she accept that her "romance" with Don is fictitious, cooked up for publicity. That evening, Don is mobbed by overly-adoring fans and escapes by jumping into the first open car he encounters; its startled driver, Kathy Selden, does not recognize him - quite a blow to his ego. Not only does she not recognize him, but she is totally unimpressed with when he reveals his star status, and unresponsive to his usual lines. A stage actress herself, she considers movie roles to be pantomime, not really acting.
Both irritated and intrigued by this encounter, Don proceeds to a party at Simpson's, where the producer unveils a short sample of a new invention: a talking picture! Everyone agrees that it is just a gadget and that Warner Brothers, which is making an entire movie with this crude device, will lose its shirt.
A huge cake is wheeled in, and when a dancing girl pops out of it Don is delighted to recognize Kathy, "the legitimate actress." Embarrassed, she nonetheless leads her showgirls in a rendition of All I Do is Dream of You. When Don teases her about her dignified career, it is more than she can bear: she tosses a cake at him, which misses Don and hits Lina. Horrified, Kathy flees the scene. Although Don follows, he is unable to catch up with her.
Three weeks later, as filming begins for Don's new picture, a story set during the French Revolution called "The Dueling Cavalier," he is feeling a little down. He has not been able to find Kathy; he feels responsible that she lost her job at the Coconut Grove, and Cosmo's remark about his films ("If you've seen one, you've seen them all"), uncomfortably resembles what Kathy said about them. Since the show must go on, Cosmo attempts to cheer up his friend with a little slapstick humor (Make 'em Laugh).
Just as shooting begins, Don learns that Lina is the one who arranged to have Kathy fired, and their actual dialogue sharply contrasts with the tender love scene they're playing.
Suddenly Simpson appears and brings the production to a halt: "The Jazz Singer," the Warner Brothers talking picture, is a huge hit. Theatre across the nation were putting in sound equipment and Monumental Studios must get on the bandwagon! "The Dueling Cavalier" must become a talking picture. When the bewildered director protests, he is told, "It's a picture. You do what you always did - you just add talking to it."
True to Simpson's prediction, musical talking pictures sweep the country (I've Got a Feeling You're Fooling; Should I Reveal Exactly How I Feel). One of the chorus girls at Monumental Studios is Kathy Selden (Beautiful Girl) and Simpson is about to offer her a small part in "The Dueling Cavalier" when Don spots her. Convinced her chances are ruined, Kathy confesses to her crime with the cake, but to her surprise, Don insists that she be hired. Simpson agrees, but insists "don't let Lina know she's on the lot." Alone with Don, Kathy admits that she is not as indifferent to him - or his movies - as she first pretended. Don arranges props and lights for a proper romantic setting and makes a confession of his own (You Were Meant For Me).
Both Don and Lina take elocution lessons to prepare for their first roles as talking actors, but Lina is making no progress at all. Because of his early vaudeville experience, Don is a natural at the vocal exercises - especially with Cosmo helping (Moses supposes). As filming starts once again, it is evident that making talkies is not going to be easy: Lina is hopeless! Remembering where the microphone has been placed is only the start of her problems.
The preview of the film is a disaster. Not only is Lina's voice as horrendous as ever, but Don's performance comes across as stilted and florid and the audience laughs uproariously. Don despairs: "I'm no actor. I never was. Just a lot of dumb show." It is Kathy who comes up with an idea: make "The Dueling Cavalier" into "The Dancing Cavalier" - a musical! Elated, they see the sun coming up and realize they've been up all night (Good Mornin'). But if the movie is a musical, what about Lina's voice? Cosmo has an idea too: dubbing! Kathy will be Lina's voice. His problems solved, Don heads for home through the pouring rain (Singin' in the Rain), happy and in love.
"The Dancing Cavalier" becomes the tale of a Broadway hoofer who dreams he is living in the time of the French Revolution. Kathy dubs Lina's songs (Would You?) without the star's knowledge, and they add an elaborate production number showing the hoofer's arrival in New York (Broadway Melody; Broadway Rhythm). Lina explodes when she discovers that Kathy's voice has been superimposed on her own and insists that the girl receive no credit in the screen; when she threatens to sue, Simpson caves in. "The Dancing Cavalier" is a colossal success at its premiere, but Kathy, Cosmo and Don's happiness is short-lived; Lina announces that Kathy's five-year contract with the studio will be spent dubbing Lina's voice without credit - and doing nothing else.
At the premiere, the audience calls for a song from Lina; this gives Don and Cosmo an idea. Lina insists that Kathy sing behind the curtain for her, and to Kathy's shock Don supports Lina's idea. Very hurt, Kathy begins Singin' in the Rain backstage as Lina mouths to the music. Don, Cosmo and Simpson raise the curtain behind Lina, revealing Kathy. Don introduces Kathy as the real star of the picture as the audience goes wild.