At Manhattan's Plaza Hotel, Karen Nash checks into suite 719, the site of her wedding night twenty-three years earlier. After chattering to the disinterested bellhop, she orders French champagne in anticipation of a romantic anniversary evening. When her husband Sam arrives, however, he is distracted with work concerns. At Karen's prompting to remember their anniversary, Sam points out with exasperation that she has both the day and the year of their wedding wrong. Soon after, Sam apologizes and kisses her, citing his current stressful work situation as the source of his bad mood. Karen offers to apply his eye drops, but when she does, he cries out in pain and snarls at her for not packing for him correctly. Sam, who has recently turned fifty-one, is vain about maintaining his youthful looks, a source of annoyance to Karen, who gazes wistfully out the window to the park below, where a young couple is kissing. Hoping to win back his favor, Karen points out that this is their honeymoon suite, but Sam proves to her that it was actually 819, causing Karen to cry. In the bedroom, she tells herself to keep her mouth shut, apologizes and hugs him, but he responds by stating that he must work. When the waiter arrives with the champagne, Sam is annoyed that Karen prattles about her family and her age so freely. Soon after, Sam's attractive secretary, Jean McCormack, arrives with papers to be signed and informs him of a major problem with the next day's presentation. As she leaves, Sam announces to Karen that he must return to the office. Karen jokes lightly about Jean to deflect her jealousy, and Sam responds defensively. Still flippant, she declares that a "fling" would be good for him, but soon grows serious and asks why they have not been happy lately. Sam answers that he longs to return to the time of his life when everything was just beginning, and Karen realizes that he is having an affair. They fight, and Sam at first tries to walk out but finally admits that he and Jean have been lovers for six months. Although he offers to do whatever Karen wants, she scoffs at him, vacillating between rage, pain and desperation. Finally, she accepts the situation, causing Sam to lash out further and accuse her of being too meek. Karen's gentle sarcasm confounds Sam, but when he tells her he will never understand her, she wonders why in that case he needs to be with someone else. As Sam walks out, Karen begs him to stay and work it out, but he tells her "maybe tomorrow" and leaves.
As Karen leaves the Plaza, famed Hollywood producer Jesse Kiplinger arrives. The smarmy, lecherous middle-aged man flirts with every woman within sight, and once in the suite, calls several women to find one who will visit him for an afternoon tryst. Finally, high school flame Muriel Tate agrees. Although they have not seen each other for fifteen years, Muriel has followed Jesse's successes closely, a fact Jesse has anticipated and is determined to use to his advantage. Despite her guilt at visiting him in his hotel room, the New Jersey housewife arrives at his door, immediately insisting that she must leave. In an all-out seduction attempt, Jesse pours on the charm, deflecting her every attempt to leave the room. Although Muriel outlines all the reasons she cannot stay for a drink, she soon accepts a stinger and the two reminisce. As she lists the many facts she has learned about him through the fan magazines, he claims never to have gotten over her, kissing her even as she is discussing her "wonderful" marriage. When Muriel rises to go, Jesse invites her to stay to meet actor Lee Marvin, and she quickly sits back down. As he makes love to her, she suddenly asks if he knows Frank Sinatra, causing him to yell in frustration. Muriel races out but he chases after her and convinces her to return. Once inside, Jesse launches into his most ambitious line, stating that his success has not quelled the loneliness of his life. Calling all three of his ex-wives "phony and unfaithful," Jesse declares that he needs Muriel to renew his faith in women. Drunk and eager to believe him, Muriel confesses that her husband is not an easy man to live with, but then prepares once again to leave. Jesse begs her to stay, and although he is initially disappointed when she asks about the Academy Awards ceremony, he mesmerizes her with a lengthy list of the stars he met there, waltzing her into the bedroom as he details the seating arrangements.
As Jesse leaves the hotel the next day, the wedding of Mimsey Hubley to Borden Eisler is about to begin. In suite 719, Mimsey has locked herself in the bathroom, while her mother Norma frantically tries to convince her to come out. Mimsey's father Roy, who obsessively counts every penny the wedding is costing him, storms into the suite and, instinctively blaming his wife, pounds on the bathroom door, exhorting Mimsey that she "can't stay in there forever, we only have the room until 6:00." As Norma screams in frustration over a ripped stocking, Roy shouts at Mimsey, whom he can see through the keyhole, to stop crying onto her wedding dress. As Roy injures his shoulder in a failed attempt to break down the bathroom door, Norma tries to stave off the impatient Eislers, who are calling from downstairs, while cajoling Roy to give her money to buy new stockings. Roy declares they must tell the guests what is happening, but Norma wonders if they can sneak away instead. Roy hugs his wife in solidarity, then tries to break the door with an expensive hotel chair. The chair is destroyed but the door, and Mimsey, remain unmoved. As a frustrated Roy climbs out onto the suite's window ledge in order to enter through the bathroom window, Norma rips Roy's coat while trying to hold him back. Kicking pigeons out of his way, he crosses the narrow ledge, reaching the bathroom window as a rainstorm starts. Norma grows hysterical first for Roy's safety and then for the state of her hairpiece, ruined in the rain. Minutes later, Roy enters through the suite's front door, announcing that he was forced to enter a neighboring room after Mimsey refused to let him in the window. Stating that no jury will convict him of murdering his daughter once they see the wedding bills, Roy hatches plans to set fire to the room to smoke Mimsey out, but Norma's pleas convince him to stop, although she breaks her diamond ring in the process. Finally, Roy entreats his daughter kindly, and she passes a note under the door asking Roy to come in. As he enters triumphantly, Norma listens jealously, and soon Roy comes out, revealing that Mimsey is afraid she and Borden will become like their parents. As the couple considers their daughter's view of them, they call up Borden, who enters the bathroom, tells Mimsey to "Cool it" and leaves. Immediately, Mimsey emerges and proceeds to the ceremony. Later, Roy and Norma watch the newlyweds leave the ceremony on a motorcycle. When Norma asks Roy if they will be all right, Roy states that Mimsey was better off in the bathroom.
