Then she went off to meet her brother, and arrived just a few minutes late. And, having just committed a double murder, and sure she’d be found out and sent to prison, she blotted it all from her mind and gave herself over to a last night of joy and consummation with her beloved brother.

But of course she was never found out. The murder-suicide scene she’d staged was good enough to pass muster, and no one ever took a good hard look at her alibi. Her friend, Sandy, kept her secret; it wouldn’t do to let out that Carolyn had been out cavorting with a boyfriend, nor would Sandy’s parents be comfortable with the knowledge that their daughter had facilitated such deception. So why not keep that little secret? Carolyn surely had enough tragedy in her life, with her father having killed her mother and himself. She didn’t need to have her sex life exposed to public scrutiny.

Nor did Billy’s alibi get much attention. He crawled into his tent after taps and crawled out of it at reveille. Case closed.

And so, on the eve of his wedding, William Thompson learned for the first time that his father was not a murderer and that his sister was.

The following day they were married.

“And lived happily ever after,” said the doctor. “A curious business, incest. More common, it turns out, than we used to think. No end of fathers, it turns out, lurch into their daughters’ beds. And they’re not always hillbillies or immigrants or welfare cases, either. It happens, as they say, in the best of families. As for brothers and sisters, well, what’s that but a childhood game carried to its logical conclusion?”