Ann Stanley (Liv Ullmann), a 40-year-old Norwegian-born divorcée who sells real estate in New York City, is on vacation in Greece when she meets 22-year-old Peter Latham (Edward Albert). They enjoy a one-night stand before she returns to America, expecting never to see him again. Back at home, where she lives with her English mother Maud Ericson (Binnie Barnes) and her seventeen-year-old daughter Trina (Deborah Raffin), she is shocked when Peter shows up at her apartment as Trina's date. But Peter isn't really interested in Trina - he loves Ann, even though she's mortified at the thought of entering an Age-Gap Romance. In a subplot, Ann's customer J.D. Rogers (Billy Green Bush) seems to be flirting with her, but is really in love with Trina.
40 Carats contains examples of:
- Age-Gap Romance: Peter and Ann, as well as Trina and Rogers. Strangely, everyone seems much more laid-back about the latter relationship than the former, despite the even more dramatic age gap.
- Disappeared Dad: Trina's father, actor Billy Boylan (Gene Kelly), is immature, irresponsible, and Married to the Job and is almost never in her life.
- Dramatic Drop: Ann drops a wine glass in response to Peter's father's hounding.
- Grade Skipper: Trina is only seventeen, but she's already done with school.
- Hiding Behind the Language Barrier: Ann and Maud discuss Trina in Norwegian, which she doesn't speak, to Trina's annoyance.
- High Heel Hurt: Ann's secretary Mrs. Margolin (Nancy Walker) pressures her to wear high heels more often, even though they hurt her feet. When most of the main characters go out dancing together, Ann's shoes cause her to be quickly exhausted, while Maud, who's wearing comfortable shoes, continues to dance energetically.
- Meet the In-Laws: Ann meets Peter's parents, who have a dysfunctional relationship with him and each other. It goes very badly - Peter's father tears into the relationship and almost convinces Ann to break up with Peter.
- Not Staying for Breakfast: The morning after Ann and Peter first have sex, Ann sneaks away from the campsite, flags down a nearby boat, and takes off while Peter sleeps.
- Shirtless Scene: In Greece, Peter swims shirtless with Ann.
- That Man Is Dead: In Greece, Ann introduces herself to Peter as Penelope Potter. In America, she tells him, "Penelope Potter is dead!"
- Your Television Hates You: Ann is trying to forget about Peter when the TV plays a Pan-Am ad, inviting people to go on vacation to Greece and saying "Go! Go! Go!" Ann snaps, "I went!" and turns the TV off.