When the play begins, Edward Chamberlayne is hosting a cocktail party without the help of his wife, Lavinia. While Lavinia was the one who wanted to have the party in the first place, she has disappeared, and Edward makes up an excuse that she is visiting an ill aunt when his party guests wonder where she has gone. A mysterious guest at his party, whom he does not recognize, strikes up an intimate conversation with him after the other guests have left. When the guest implies that this could be good for the marriage, Edward makes it abundantly clear that he wants Lavinia back.
At this point, the guest makes a promise that he will bring back Lavinia, but that Edward is not to ask her any questions about what happened to her while she was gone. After the guest leaves, it is revealed that Edward is having an affair with one of his other guests, the young Celia. However, they have decided to end the affair after his separation from Lavinia.
In the second act, we learn that the unidentified guest was actually Sir Henry Harcourt-Reilly, a therapist Edward and Lavinia have been seeing for some time. In this act, they have a joint session and have an open discussion about the realities of their relationship and ultimately decide to give it another try. After they leave, Celia comes to visit Reilly, and tells him that she wants to pursue a spiritually elevated life, rather than an ordinary and predictable one. He decides she is up to the task and has a car arranged to pick her up and bring her to a special kind of sanatorium.
Act 3 takes place two years later, and Lavinia and Edward are having a cocktail party together. They are joined by their old friends. Before the party officially begins, Alex, a friend of theirs who has been working on a British colonial island in Asia, tells the group that Celia was killed by the "heathen natives" on the island, crucified on an anthill. Everyone mourns her death, but decides that she chose that fate and that her sacrifice was a noble one. The play ends with Lavinia and Edward waiting for their party to begin.