Therese Belivet is a lonely young woman, just beginning her adult life in Manhattan and looking for a chance to launch her career as a theatre set designer. When she was a young girl, her widowed mother sent her to an Episcopalian boarding school, leaving her with a sense of abandonment. Therese is dating Richard, a young man she does not love and does not enjoy having sex with. On a long and monotonous day at work in the toy section of a department store during the Christmas season, Therese becomes interested in a customer, an elegant and beautiful woman in her early thirties. The woman's name is Carol Aird and she gives Therese her address so her purchases may be delivered. On an impulse, Therese sends her a Christmas card. Carol, who is going through a difficult separation and divorce and is herself quite lonely, unexpectedly responds. The two begin to spend time together. Therese develops a strong attachment to Carol. Richard accuses Therese of having a "schoolgirl crush," but Therese knows it is more than that: she is in love with Carol.
Carol's husband, Harge, is suspicious of Carol's relationship with Therese, whom he meets briefly when Therese stays over at Carol's house in New Jersey. Carol had previously admitted to Harge that she had a short-lived sexual relationship months earlier with her best friend, Abby. Harge takes his and Carol's daughter, Rindy, to live with him, limiting Carol's access to her as divorce proceedings continue. To escape from the tension in New York, Carol and Therese take a road trip West as far as Utah, over the course of which it becomes clear that the feelings they have for each other are romantic and sexual. They become physically as well as emotionally intimate and declare their love for each other.
The women become aware that a private investigator is following them, hired by Harge to gather evidence that could be used against Carol by incriminating her as homosexual in the upcoming custody hearings. They realize the investigator has already bugged the hotel room in which Carol and Therese first had sex. On a road in Nebraska, after the detective has followed them for miles and clearly intends to continue doing so, Carol confronts him and demands that he hand over any evidence against her. She pays him a high price for some tapes even though he warns her that he has already sent several tapes and other evidence to Harge in New York. Carol knows that she will lose custody of Rindy if she continues her relationship with Therese. She decides to return to New York to fight for her rights regarding her daughter and will return to Therese as soon as she can. Therese stays alone in the Midwest; eventually, Carol writes to tell her that she has agreed not to continue their relationship.
The evidence for Carol's homosexuality is so strong that she capitulates to Harge rather than having the details of her behavior aired in court. She submits to an agreement that gives him full custody of Rindy and leaves her with limited supervised visits.
Though heartbroken, Therese returns to New York to rebuild her life. Therese and Carol arrange to meet again. Therese, still hurt that Carol abandoned her in a hopeless attempt to maintain a relationship with Rindy, declines Carol's invitation to live with her. They part, each headed for a different evening engagement. Therese, after a brief flirtation with an English actress that leaves her ashamed, quickly reviews her relationships—"loneliness swept over her like a rushing wind"—and goes to find Carol, who greets her more eagerly than ever before.
