Cobbett Stoddard, a concert pianist in late middle age, is the overnight guest of Percival and Elizabeth Hankins. Elizabeth, nee Ferry, had been Cobb's piano pupil. At the time, he'd been unhappily married. After initiating a brief, perplexing sexual relationship with him, she had disappeared from his life without explanation.

Seeing her for the first time in the intervening eighteen years, Cobb finds that Elizabeth is unwilling to acknowledge their former intimacy in any way. Her husband, who has inherited family wealth and the handsome estate on which the Hankins family lives, strikes Cobb as an eccentric. But Cobb is secretly captivated by the beauty and vitality of Amy, Elizabeth's seventeen-year-old daughter.

Unable to sleep after his evening concert performance on this oppressively hot, moonlit September night, Cobb steals from the guestroom to a treadle swing in the garden. Here, unobserved, he witnesses Amy and her boyfriend making love in a souped-up jeep when they return from a date. Cobb has been crushed that Amy, like her parents, chose not to attend his recital.

The next morning, Percival Hankins, who is twenty-some years older than his wife, gives Cobb an account of their courtship and marriage. Cobb is startled to realize that at that very time, Elizabeth had been carrying on the affair with him.

With a melancholy sense of aging, Cobb recognizes that he might never again experience the passion Amy has aroused in him. As he leaves the Hankins estate, he catches one last glimpse of her running through the rain to and from the mailbox. Suddenly the idea that he might be the father of his former lover's daughter pops into his head. He'll never know.