It stars magazine editor Larry Blake (Douglas), who is on vacation at a ski resort in Idaho. He shows no interest in the sport until he spots the beautiful ski instructor Karin (Garbo) and asks her for private lessons. They slowly start to get along and after being snowed in together, Larry's panicked secretary Miss Ellis (Ruth Gordon) and friend Mr. Miller (Roland Young) discover that Larry is fine and married to his skiing teacher.
The newlyweds appear to be an idyllic romantic couple but cracks start to show immediately when Larry is wanted back in New York. They bicker through the night and into the next morning with Karin accusing Larry of being two-faced. Offended, Larry promises her that he will return to her when his business trip is done as he leaves their home.
Soon after Larry is back in New York, Karin tells Miss Ellis through a telegram that she wants to surprise him. When she sneaks into a Broadway rehearsal he's watching, Karin notices how much her husband gets along with his colleague Griselda Vaughn (Bennett) and decides to leave, but as she and Miss Ellis try to sneak out, they are caught by Mr Miller. In a blind panic, they tell him that Karin is actually Katherine — Karin's outgoing and flirtatious identical twin sister, who intends to seek out what her twin sees in her brother-in-law Larry Blake.
This film provides examples of:
- Book Ends: The movie begins and ends with Larry and Karin skiing together, with Larry slipping about.
- Can't Hold Her Liquor: Karin is not used to champagne and uses this to try and scare Larry away from women like Katherine. Unfortunately, he still wants to see Katherine again and Karin has the worst hangover, to the point of wearing a bandage around her head like she had mumps.
- Dancing Royalty: Karin is an unintentional one. In her disguise as Katherine, she tries to convince doubters at a nightclub that she's a dancer. She gets on the dancefloor, improvises moves to a stunned audience, and soon enough, everyone watching is enthusiastically imitating her, turning it into a flash mob. The Betty to her Veronica is absolutely furious.
- Dramatic Irony: After meeting Katherine, Larry phones the ski resort to speak to his wife. Just as he thought, his wife left for New York days ago.
- Fake Twin Gambit: Karin pretending to be Katherine, in order to get closer to her husband without him suspecting.
- Femme Fatale: Karin (as Katherine) tries to tempt Larry into cheating on her sister. It almost happens, much to Karin's surprise.
- Foreshadowing: When Larry collects Katherine's napkin from under the table, he notices her painted toenails.
- Nice Girl:
- Karin. It's a reason why she's suspicious of her husband, who she assumes prefers outgoing and sexual women. Outside of pretending to be her "twin sister", she's never cold or mean to suspected mistresses.
- Miss Ellis, who respects people's wishes, no matter how dishonest, but is never cruel.
- Shout-Out: Larry tells Katherine to look up "alone" in the dictionary. This is a reference to numerous Greta Garbo films in which her characters reference being alone or "wanting to be alone."
- Sickeningly Sweethearts: Larry and Karin when Miss Ellis and Mr Miller discover they're alive and well. They don't hold back on passionately kissing each other, wistfully talking about the time they've spent together, and even roll around on the floor in front of their guests.
- Virgin–Whore Complex: Karin (the nice girl) vs her flirtatious "twin sister" Katherine.