The setting is Sweden, around the year 1900. Middle aged Fredrik Egerman is a successful lawyer. He has recently married an 18-year-old trophy wife, Anne, a vain girl who is in love with Fredrik, but too immature to grasp the concept of marriage. The two have been married for eleven months, but Anne still protects her virginity. Fredrik laments his inability to make love to his wife. Meanwhile, his son Henrik, a year older than his stepmother, is feeling extremely frustrated. He is a seminary student and everyone is always teasing him, never taking him seriously or letting him talk. Anne is intrigued by him, but fails to understand his real meaning. Anne promises her husband that she will consent to have sex shortly. Anne's maidservant Petra, an experienced and forthright girl, slightly older than the teen herself, offers her worldly but crass advice.
Desiree Armfeldt is a prominent and glamorous actress who is now reduced to touring in small towns. Madame Armfeldt, Desiree's mother, has taken over the care of Desiree's daughter Fredrika. Fredrika misses her mother, but Desiree continually puts off going to see her, preferring, somewhat ironically, "The Glamorous Life". She is performing near Fredrik's home, and he brings Anne to see the play. While there, Desiree notices Fredrik; the two were lovers years before. Anne, suspicious and annoyed because of Desiree's amorous glances, demands that Fredrik take her home immediately. Meanwhile, Petra has been trying to seduce Henrik.
That night, as Fredrik remembers his past with Desiree, he sneaks out to see her; the two share a happy but strained reunion, as they remember. They reflect on their new lives, and Fredrik tries to explain how much he loves Anne. Desiree responds sarcastically, boasting of her own adultery, as she has been seeing the married dragoon, Count Carl-Magnus Malcolm. Upon learning that Fredrik has gone for eleven months without sex, she agrees to accommodate him as a favor for an old friend.
Madame Armfeldt offers advice to young Fredrika. The elderly woman reflects poignantly on her own checkered past, and wonders what happened to her refined "Liaisons". Back in Desiree's apartment, Count Carl-Magnus Malcolm proclaims his unannounced arrival in his typical booming voice. Fredrik and Desiree fool the gullible Count into believing that their disheveled appearance was entirely innocent, but he is still suspicious. He instantly dislikes Fredrik and returns to his wife, Countess Charlotte. Charlotte is quite aware of her husband's infidelity, but Carl-Magnus is too absorbed in his suspicions of Desiree to talk to her. When she persuades him to blurt out the whole story, a twist is revealed—Charlotte's little sister is a school friend of Anne's.
Charlotte visits Anne, who is talking with Petra. Charlotte describes Fredrik's meeting with Desiree; Anne reacts with shock and horror. The older woman explains to Anne that such is the lot of a wife, and that marriage brings pain. Meanwhile, Desiree asks Madame Armfeldt to host a party for Fredrik, Anne, and Henrik. Though reluctant, Madame Armfeldt agrees. She sends out a personal invitation; its receipt sends the women into a frenzy, imagining a weekend in the country. Anne does not want to accept the invitation, but Charlotte convinces her to do so to heighten the contrast between the older woman and the young teenager. Meanwhile, the Count has plans of his own — as a birthday present to his wife, the pair will attend the party uninvited. Carl-Magnus plans to challenge Fredrik to a duel, while Charlotte hopes to seduce the lawyer to make her husband jealous and end his philandering. The day of the party dawns.
Armfeldt's country estate is bathed in the golden glow of perpetual summer sunset at this high latitude. Everyone arrives, each carrying their own amorous purposes and desires—even Petra, who catches the eye of Armfeldt's fetching manservant, Frid. The women begin to act against each other. Fredrik is astonished to learn the name of Desiree's daughter. Henrik meets Fredrika, and confesses his deep love for Anne to her. Meanwhile, in the garden, Fredrik and Carl-Magnus reflect on how difficult it is to be annoyed with Desiree, agreeing it would have been wonderful had she not been quite so wonderful. Dinner is served, and the characters' perpetual anticipation enlivens that meal.
At dinner, Charlotte attempts to flirt with Fredrik, while Anne and Desiree trade insults. Soon, everyone is shouting and scolding everyone else, except for Henrik, who finally stands up for himself. He shrieks at them for being completely amoral, and flees away. Stunned, everyone reflects on the situation and wanders away. Fredrika tells Anne of Henrik's secret love, and the two dash off searching for him. Meanwhile, Desiree meets Fredrik and asks if he still wants to be "rescued" from his life. Fredrik answers honestly that he loves Desiree, but only as a dream. Hurt and bitter, Desiree can only reflect on the nature of her life. Anne finds Henrik, who is attempting to commit suicide. The clumsy boy cannot complete the task, and Anne tells him that she has feelings for him, too. The pair begins to kiss, which leads to Anne's first sexual encounter. Meanwhile, not far away, Frid sleeps in Petra's lap. The maid thinks of the joy and freedom that she longs for before becoming trapped in marriage. Henrik and Anne, happy together, run away to start their new life. However, Carl-Magnus is enraged by this and attempts to shoot the lovers, but Desiree and Charlotte prevent him, while lamenting both the pains of marriage and the strange behavior of married people. With Carl-Magnus calmed, Charlotte confesses her plan to Fredrik, and the two commiserate on a bench. Carl-Magnus, preparing to romance Desiree, sees this and challenges Fredrik to Russian Roulette, at which a nervous Fredrik misfires and simply grazes his own ear. Victorious, Carl-Magnus begins to romance Charlotte, granting her wish at last.
After the Count and Countess leave, Fredrika and Madame Armfeldt discuss the chaos of the recent turns-of-events. The elderly woman then asks Fredrika a surprising question: "What is it all for?" Fredrika thinks about this, and decides that it must be worth it. Madame Armfeldt is surprised, ruefully noting that she rejected love for material wealth at Fredrika's age. She praises her granddaughter and remembers true love's fleeting nature.
Fredrik finally confesses his love for Desiree, acknowledges that Fredrika is his daughter, and the two promise to start a new life together. Armfeldt sits alone with Fredrika. Fredrika tells her grandmother that she has watched carefully, but still has not seen the night smile. Armfeldt laughs and points out that the night has indeed smiled twice: first on Henrik and Anne, the young, and second on Desiree and Fredrik, the fools. As the two wait for the third smile, on the old, it happens: Madame Armfeldt closes her eyes, and dies peacefully with Fredrika beside her.
