An adulterous couple discovers that their children are sexually involved.

Self-made millionaire research chemist Ken Jorgenson, a former lifeguard at the exclusive summer community on Pine Island, returns there twenty years later as a guest with his wife Helen and daughter Molly. Ken reserves rooms at the home of jaded alcoholic Bart Hunter, who has lost the fortune built by his father. Bart has turned his mansion into an inn and thinks that Ken, who is a Swedish immigrant's son, is coming to gloat about their respective reversal of fortunes. Although Bart wants to refuse the Jorgensons the reservation, his wife Sylvia insists that they are too much in debt to turn down paying customers. The Jorgensons arrive on the island by chartered yacht, a pretense insisted upon by Helen, who is obsessed with appearances and status. An acrimonious woman who no longer sleeps with Ken, Helen tries to repress teenaged Molly's budding sexuality and has burdened the family with her excessive bigotry, making it difficult for the family to settle anywhere. At dinner, the boorish Bart tries to titillate Helen by describing his gardens as an "aphrodisiac." To smooth over Helen's obvious indignation, Sylvia explains that the island has a history of summer romances. After dinner, the Hunters' son Johnny takes Molly on a moonlit tour of the gardens, where she explains how she took kissing lessons from a boy at her high school. When she kisses Johnny goodnight, the nosy groundskeeper, Todd Hasper, sees them and reports to Helen, who then blames Ken's Swedish genes for her daughter's interest in sex. Although Ken accuses Helen of suffocating Molly's "natural instincts," Helen scolds Molly and tells her that "wanting a man is cheap." Molly has a heart-to-heart talk with Ken, asking him why he married a woman who does not love either of them. Ken says they married out of loneliness and admits he once loved someone, but had nothing to offer her. He tells Molly that the sole reason for existence is to love and be loved. He is sorry that Helen does not know how to love and that he could not teach her. One rainy day, Bart's godmother, Mrs. Hamilton Hamble, complains to Sylvia about the leak in her water closet. When Sylvia says she has called the mainland for a plumber, Mrs. Hamble, acknowledging Bart's uselessness, asks Ken to go into the attic and check it. As he and Sylvia climb the attic stairs, Sylvia confesses that she has delayed calling a professional, because Bart fears the roof has a structural problem that might result in the building being condemned. After determining that the roof is sound, Ken asks why she has been avoiding him. Twenty years ago, they had been lovers, but Sylvia's mother had arranged her marriage with the then-wealthy Bart. Ken says he got married a week after seeing Sylvia's wedding picture in the newspaper. Both admit to living a "half-life" without love, which they have maintained for the sake of their children, and realize they are still in love with each other, but do not want to hurt others. When they go downstairs, Mrs. Hamble warns Sylvia that she could hear their entire conversation through the circulation vents in her room. Recalling how Bart was "plastered" at his own wedding, Mrs. Hamble notes that he has rarely been sober and suggests "straight talk." She lays out Sylvia's options: divorce, a well-planned and discreet affair, or an unplanned, careless affair that courts gossip. At two in the morning, Sylvia and Ken meet in the boathouse. Fearing they could lose their children, neither wants to risk divorce, and so they concede that at least they will have this summer together. When they part before sunrise, they do not realize that Todd has discovered their tryst. Soon after, Helen, pleased to receive damning reports about Ken from Todd, calls her scheming mother. Together they strategize how to trap Ken in the act of infidelity, so that Helen can divorce him and receive a big settlement. When Johnny and Molly go sailing one day, bad weather capsizes their boat, stranding them on an uninhabited island. After they are rescued, Helen calls a doctor to examine Molly for signs of lost virginity, despite the girl's protestation of innocence. Afterward, Molly runs away, causing Johnny to threaten Helen, who complains to the police. When a mainland policeman checks into Molly's disappearance and Helen's complaint, Ken, who has just returned from a short business trip, sides with Johnny. Vindictively, Helen blurts out that Ken and Sylvia are sleeping together. Seeing Johnny's pain and shock, Sylvia accuses Helen of trying to destroy the children, and Bart unsympathetically orders Helen to leave the island immediately. To Sylvia, Bart admits that he has known about her love for Ken for twenty years, but was "fascinated" by her "front" as wife and mother. Although he is willing to continue their marriage, she wants to end it. Molly is soon found, and after both couples divorce, Helen slanders Ken and Sylvia, and her accusations are repeated in the newspapers. Because of the scandal, the respective divorce settlements award the children to Helen and Bart, who both send their children to exclusive boarding schools. Embarrassed by the scandal and hating their parents, Johnny and Molly feel alone against the world and write to each other. Helen finds and reads Johnny's letters and warns Molly that there is "bad blood" in the son of a "drunkard and a harlot." Throwing the letters into the fire, Molly accuses Helen of making the only thing she has "to live for" into something "dirty." Although strictly forbidden to see each other, Molly and Johnny arrange to meet secretly at a church during Christmas break. When they kiss in greeting, Molly is spotted by a gossipy friend and her mother, who reports back to Helen. Later, Helen slaps Molly, knocking over their Christmas tree, thus alienating her daughter. When Ken and Sylvia marry, they are disappointed that the children do not attend. Ken visits Molly at her dormitory, inviting her to visit their beachside house during spring break, pleading that they need each other. Helen tries to block the court order allowing Ken to see Molly one month a year, but her lawyer warns her that Ken would then be able to stop paying her alimony. When Molly comes to visit Ken at spring break, and Johnny arrives a few days later, the young people shun their parents and spend their time in beach hideaways. One night, they tell Ken and Sylvia they are going to see the movie King Kong , but instead, make love in an oceanside lookout. When they return near daybreak, Ken and Sylvia discuss ways to talk to them about the dangers of love, without destroying the beauty of it. The next morning, when Ken tries to caution Molly, she bristles at his meddling. Weeks later, Molly calls Johnny to say she is pregnant. He hitchhikes from school to be with her and they pawn her fur coat, a gift from Ken, for money to travel to Pine Island. Although Johnny expects his father to be "open-minded" enough to give them permission to marry, Bart, suffering from ulcers and alcoholism, says they are too young. After the Coast Guard takes Bart to a Boston naval hospital, he calls Helen and the police. Molly and Johnny ask a justice of the peace to marry them, but he refuses, because they cannot prove they are of age. Meanwhile, Helen calls Ken to say that Bart wants to bring charges against them in juvenile court until they "cool off." Having nowhere else to go, Molly and Johnny ask for help from Ken and Sylvia, who give their support. Later, newlyweds Molly and Johnny return to Pine Island to live.