In London, architect Robert Blakeley and his mercurial wife Zee attend a swank party where Robert is attracted to Stella, a pretty young widow and dress designer. Immediately suspicious of Robert’s interest, Zee intrudes on the pair to make sarcastic remarks. Although maintaining a calm exterior, Stella is disturbed both by Robert’s overt attraction to her and Zee’s caustic observations, and bolts for the bathroom to weep. Robert follows to ask for a date and the couple meets the following evening. Amused by but also wary of Robert’s intensity, Stella obliquely asks about his relationship with Zee, which Robert describes as “false.” Soon after, Robert and Stella become lovers. Disturbed by the fervor of Robert’s infatuation, Zee visits Stella’s boutique where she boldly declares that her husband prefers earthy, well-rounded figures like Zee’s and that she and Robert have always been extraordinarily close. Stella remains unflustered by Zee’s bombastic verbal assault and even agrees to her demand to sell her a dress made for another customer. Over several weeks, Robert and Stella continue their affair and when at home with Zee, Robert endures her continual mocking and baiting. When Zee reminds Robert that she is fully aware of his numerous affairs with secretaries in his office and that Stella is no different, Robert tells her to find a lover and leave him in peace. Undaunted, Zee points out that Robert has long been attracted to her impulsive, reckless nature, something which the sedate Stella lacks. After Zee hints that she had a brief fling with her physician that very afternoon, she is pleased when Robert responds with angry jealousy, then theatrically announces that she is going to Spain to rest. Robert spends the next evening with Stella and meets her nine-year-old twin sons Oscar and Shaun, on brief holiday from school. Confident that Zee will be in Spain, Robert invites Stella to his flat the following evening for a romantic dinner. The next afternoon at work, however, Robert is furious when his mousy young secretary Rita announces that Zee is waiting in the front office and expecting Robert to drive her home. Indignant that Zee has spent so much money to fly to Spain and back for a single night, Robert chastises her, but Zee complains that the weather was cold and wet. When Robert makes several attempts to invite Zee to dine at a restaurant, she insists on returning home. Upon finding their maid has left a dinner set-up for two and a note about Stella, Zee gleefully ridicules Robert, then boisterously welcomes the startled Stella moments later. Hoping to dull Zee’s barbed attacks, Robert finally suggests they dine out, but when Zee continues her derision of Stella at the restaurant, an exasperated Robert leaves. Although Stella is amused by Robert’s departure, Zee is surprised and for a few moments the women speak candidly. Zee observes they might have been friends in another situation, but Stella states she is not who she appears and admits she was expelled from school when younger. A few nights later, Robert goes over the household bills in rising frustration while Zee casually dresses for a party. Angered, Robert binds Zee’s hands behind her and forces her to sit with him in the office to discuss their finances. When Zee maintains her usual taunting response, he grows livid, but cannot resist Zee’s manipulations and eventually the couple makes love. Soon after, Zee meets with her homosexual friend Gordon to express concern that Robert may truly be in love with Stella. Scorning Stella’s sensitivity and guilelessness, Zee asks Gordon to help her dig up information to use against Stella. When Gordon notes her callousness, Zee claims she can do whatever necessary to protect her relationship with Robert. Disgruntled by Gordon’s hesitation, Zee drives to Stella’s flat and throws garbage cans against the wall and is satisfied when Robert peers down at her angrily. Distressed, Stella admits to Robert that she is afraid of Zee’s obsessive nature and wonders if the couple will ever find peace. To soothe her, Robert offers to take her to Scotland and the couple depart the next day. The night after a wonderful day alone together, however, the couple receives a phone call from Gordon reporting that Zee has been in a car accident. Dismayed and wondering how Zee knew where he and Stella were, Robert promises Stella they will look for a flat together, then reluctantly returns to London. Upon finding an apparently healthy Zee, Robert quarrels with her and Zee fears she may have gone too far. The next day, Stella and Robert find a flat, but when she wonders aloud what it will be like living with Robert daily, he grows distant. That evening at home, Robert finds Zee at home uncharacteristically quiet and withdrawn. The couple talks about never having had children and their inability even to keep pets. Zee ruefully reflects they should never have gone to the party where Robert met Stella and retires. Later that night, Robert is awakened by the sound of running water and must break down the bathroom door to reach Zee, who has slashed her wrists and is unconscious in the running bath. After taking Zee to the hospital, Robert visits Stella at the new flat and observes that Zee must love him a great deal, then confides that it crossed his mind to allow Zee to die. Robert then tells Stella that Zee asked her to visit at the hospital. When Stella expresses surprise, Robert points out that Zee genuinely likes Stella and admires her serenity. Although Stella declares she has grown to hate what others expect her to be, she nevertheless visits Zee, whom she finds unusually composed. Zee matter-of-factly describes how difficult it has been for her while Robert has been seeing Stella and observes that people always find their happiness at the expense of others. Moved to offer a similar confidence, Stella reveals that she was expelled from school for falling in love with a nun, then asks if Zee had planned the suicide long before. Zee admits it was an impulsive act, prompted by learning from a friend that Stella and Robert had rented the flat together. The women continue their candid exchange of confidences and end the visit with an embrace. Some days later, while unpacking at the new flat, Robert tells Stella that Zee is throwing him a small farewell party that night. Instead of the intimate gathering Zee had implied, however, the party is a lively, crowded affair and Robert quickly becomes drunk. Zee hastily takes him to her bedroom and the next morning Robert refuses to acknowledge having had sex with her. When Zee protests, Robert demands that she stop playing games with people and calls her a phony. Zee replies by pointing out that Stella is the biggest phony of them all, but Robert storms out, refusing to hear anything further. At the new flat, Stella is distressed that Robert has stayed out all night and laments that Zee will always be a presence in their lives. In frustration Robert goes to a nearby bar and after getting drunk telephones Rita, who eagerly agrees to meet him. Meanwhile, Zee goes to the new flat to visit Stella, who grows uncomfortable when Zee suggests that she knows Stella’s secret and what she wants. That evening, Robert returns to the flat from Rita’s and is surprised to find Zee. Discovering Stella in bed naked, Robert asks what happened, but Stella claims she does not know. Zee assures Robert that Stella will be confused for a brief period, but will be fine in a few days and demands he take her to dinner.