Singin’ in the Rain begins back in the roaring 20s with the popular silent film star Don Lockwood. Don is paired with a very vain and petty leading lady at Monumental Pictures named Lina Lamont. The studio links them romantically in the gossip columns of the day to increase their popularity, and Lina believes them to be an actual couple. Don, on the other hand, barely tolerates the woman.
At the premiere of Don’s latest film, The Royal Rascal, he begins to tell the crowd his exaggerated life story; much to the crowd’s delight. The go to see the film, and once it is over, Don looks to escape from his overenthusiastic fans. He hops into Kathy Selden’s car as she passes by him. The two engage in a they snippy conversation where Kathy claims to be a stage actress herself; disapproving of his undignified accomplishments as a movie star as she drives him home.
Later, Monumental Pictures’ head, R.F. Simpson shows a short demonstration of a talking picture at a party. Nobody there is terribly impressed though. A fake cake comes out, and stops before Don. Kathy pops out of it; much to his amusement. He teases Kathy about being a chorus girl after she belittled him in the car over being a movie star.
Furious, Kathy grabs real cake, and hurls it at him, but misses. It hits Lina squarely in the face, and Kathy runs off. Over the next few weeks, Don searches high and low for Kathy as he now has a bit of a crush on the woman. After Lina reveals that she had Kathy fired, Don is able to locate her at another Monumental Pictures production. Once cornered, Kathy reveals that she’s actually a big fan of his.
Soon a rival movie studio releases its first talking picture, 1927’s The Jazz Singer. As its popularity explodes, R.F. has no choice but to make his next Lockwood/Lamont film called The Dueling Cavalier into a talkie.
However, the production is beset with a number of difficulties; including getting past Lina’s grating New York accent. Even a diction coach fails to improve her speaking voice. A screening for the film proves to be an equal disaster due to the actors being barely audible from poorly placed microphones.
During one spot in the film, Don repeats the line “I love you” over and over to Lina. The result is uncontrolled laughter from the audience. It gets even worse once the sound becomes out of sync from the film.
However, Don, Kathy, and Don’s pal, Cosmo are undaunted. They come up with a bright idea to turn The Dueling Cavalier into the musical The Dancing Cavalier. However, Lina’s horrid voice remains a huge problem. Cosmo suggests that they use Kathy’s voice in place of Lina’s. R.F. approves the plan, but orders everyone not to tell Lina what they are doing.
Eventually, Lina finds out, and of course, she’s pissed about it. When she finds out R.F. plans to give Kathy screen credit and a big publicity push afterwards, she hits the roof. Lina threatens to sue R.F. and anyone else who gets in her way unless Kathy continues working as her uncredited voice. R.F. gives in due to a clause in her contract states that the studio is responsible for media coverage of her, and she can sue if she is not happy with it.
The Dancing Cavalier premiers to tremendous acclaim, and the adoring fans in attendance shout for Lina to sing live. Don, Cosmo, and R.F. order her to lip sync into the microphone while a concealed Kathy sings into a second one behind a curtain. Lina agrees, and while she performs for the crowd, the three men raise the curtain to reveal Lina as a fraud.
Horrified, Lina runs off. Kathy is equally horrified, and she tries to run off too, but Don announces to the audience that she’s the film’s real star. Later, Kathy and Don share a kiss in front of a billboard for their new film titled Singin’ in the Rain.