Lewis Dodd (Boyer) is a failed composer, and after a negative review, he goes to heal his wounds by visiting his friend and musical mentor Albert Sanger. His home is located in the Swiss Alps, and with him are his daughters to keep him company. These include: Paula (Joyce Reynolds), Tessa (Fontaine), and Toni (Brenda Marshall). They all love Lewis very much, but Tessa has a special place for him in her heart even though shes only 14 years old. Lewis, in turn, treats her like he should, as a young girl.
Tragedy strikes when Albert dies, and Lewis writes to the girls rich English relatives, the Creightons. Their uncle, Charles Creighton (Coburn), and his daughter, Florence (Smith), come to handle the affairs and take the girls back to London. In those few weeks, Lewis falls in love with Florence, and she with him, causing much pain to Tessa.
Half a year later, and the marriage between Florence and Lewis is very much on the rocks.
Both Paula and Tessa are put in an English boarding school—eventually running away from it—, and Florence has grown to resent the affection Lewis has for the girls, especially Tessa's deep attachment to Lewis.
All things come to a head with Lewis' new musical piece; much to the chagrin of Florence, Lewis follows Tessas advice over her own and so begins an ugly competition.
Peter Lorre has a small supporting role as Fritz, whom Toni marries. Joan Fontaine considered The Constant Nymph one of her best performances, alongside Letter from an Unknown Woman which is something of a companion piece to this film, as they share many similarities. This film also gave Fontaine her third and final Academy Award nomination for Best Actress.
- All Love Is Unrequited: Tessa loves Lewis, but he doesnt see her in that way. Justified, since shes only 14 years old. And eventually, this happens to Florence once Lewis finds out he loves Tessa.
- Anguished Declaration of Love: A heartbreaking one from Tessa to Lewis where he finally realizes that he loves her.
- At the Opera Tonight: Tomorrow, Lewis symphony, plays in a London opera house.
- Bratty Teenage Daughter: Toni runs away with Fritz, only to come back and reject him, and then eventually marry him.
- Dawson Casting: Joan Fontaine — who turned 26 the year of the film's release — plays the 14-year-old Tessa.
- Death by Despair: When Tessa realizes that Lewis piece has no melodic line, and therefore, hasnt learned to feel.
- Downer Ending: Someone dies. Tessa dies while hearing Lewis symphony, Tomorrow, because he hadn't found true feeling & has been pray to Florences influence. Now that Tessa has died, he can create something meaningful. The films ends with the Anvilicious shot of a fire growing and growing to prove this notion.
- Genki Girl: Running around wildly without any shoes, and being full of energy, despite her heart problem, is Tessas MO.
- Ill Girl: Severe stitches hurt Tessa, and she often faints due to heart problems. In her boarding school, she's not allowed to play hockey because of her condition. This foreshadows her eventual death.
- I Want My Beloved to Be Happy: Two examples: first, after Tessa makes her Anguished Declaration of Love to Lewis, she tells him that they cant be together because of his marriage, even though they both know love each other very much. And a surprising change from Florence, who realizes that Tessas love is true, and she wants Lewis to find happiness, even if it means being with someone else. Unfortunately, this reconciliation comes all too late.
- The Film of the Book: Based on the 1924 novel by Margaret Kennedy of the same name.
- Horrible Judge of Character: Thinking that she could enter Lewis cloud of "misunderstood artist", Florence believes she can change him to be more like her. Lewis, likewise, believes that Florence will understand him and his ways.
- Horned Hairdo: Florence wears very severe looking victory rolls.
- Jerk with a Heart of Gold: Lewis at times: hes very sweet to the girls, but can often be a downright jerk to Florence.
- Longing Look: Tessa gives countless to Lewis. He doesnt notice them at all.
- Love Triangle: A rather strange, and even Squick-worthy one to some, between Lewis, Florence, and Tessa.
- Mouthy Kid: Florence abhors the way the girls seem to be more adult in their way of speaking, but have yet to gain more maturity.
- Oblivious to Love: And how! Lewis doesnt realize the very obvious affection that Tessa shows to him.
- One True Love: In a fantasy/dream Tessa has, she tells Lewis that they were made for each other, that she was always going to be with him, but fate made her a few years late in finding him, but young or old, they were meant to be together.
- Star-Crossed Lovers: Tess and Lewis.
- Wise Beyond Their Years: Tessa fits this trope perfectly. She shows some immaturity at times, but when it comes to her love for Lewis, shes very intuitive. She even knows that she must try and control herself because hes married and is greatly offended when Florence implies that she has impure (read: sexual) feelings for Lewis.