The curtain rises on a smart London cocktail party at the home of Edward Chamberlayne. His wife, Lavinia,has arranged it but is absent. Edward later confides to "the unidentified guest" that Lavinia has left him. On learning that the marriage has lasted five years and there are no children, the guest at first tries to persuade Edward that the parting is a good thing, but in the face of Edward's determination to get her back, promises to bring her the next day. He then leaves. Another guest, Peter, returns to beg Edward to intercede for him with yet another guest, Celia, with whom he is in unreciprocated love. Edward agrees. In the next scene we learn that Edward and Celia have been lovers for some time. They re-consider their relationship in the light of Lavinia's departure and decide to end it. The next day Lavinia returns and the guests reassemble in somewhat mysterious circumstances (except the unidentified one who visits briefly before the others arrive).
In Act Two we learn that the mystery guest is an eminent psychiatrist. He sees first another guest from the party, who has steered Edward to the psychiatrist. Then he has a long conversation with Edward in which Edward reassesses the value of his relationship with his wife. She then enters. She is already a patient. They discuss their relationships with each other and with other partners and decide with great hesitation to go home together. Celia is then seen and admitted to the sanatorium. It is for the very well not the sick.
Act Three is set at a cocktail party at the Chamberlaynes two years later. They have plainly found a modus vivendi - perhaps happiness, certainly contentment. Celia went to do good works in Africa but dies near an ant-hill. The psychiatrist explains that that was her destiny. Otherwise all seem destined to live more or less happily ever after.