Edward Chamberlayne's wife Lavinia has left him, after five years of marriage, just as they are about to host a cocktail party at their London home. To keep up appearances, he pretends that she has gone to see her aunt. Later, he confesses to a mysterious Unidentified Guest that Lavinia has in fact left him. The Unidentified Guest offers to bring Lavinia back, and does so.
The Unidentified Guest turns out to be the 'psychiatrist' Sir Henry Harcourt-Reilly. Edward and Lavinia both consult with Reilly at his office. He tells them that they have been deceiving themselves and must face life's realities: their life together, though hollow and superficial, is preferable to life apart. This message is difficult for the play's third main character, Celia, to accept. She, at the psychiatrist's urging, sets out upon the path to sainthood, embracing a life of greater honesty and salvation that leads her to become a Christian mystic fated to endure martyrdom on the fictional Eastern island of Kinkanja. Following Celia's consultation with the 'psychiatrist', it is revealed that the characters Reilly, Julia, and Alex are not, in fact, humans but angelic beings dedicated to the 'transhumanising' of the human soul: two paths lie open to humans: the first being the way of companionable self-deception ('the hearth') embraced by the vast majority – as epitomised in the relationship between Edward and Lavinia, and the second that of the saint, embraced by a gifted — or burdened — few.
Two years later, Edward and Lavinia, now better adjusted, host another cocktail party, at which they are told by Alex of Celia's martyrdom, for which they confess feeling a measure of guilt at what they consider the tragic waste of her life, but which Reilly considers a triumph. It is further hinted that Peter Quilpe is another of the rare individuals destined, like Celia, to follow the arduous but fulfilling path to sainthood/enlightenment.
