Quite a while back, I wrote down a bunch of "plot bunnies" using the common idea of someone walking in on two other people having sex and reacting by joining in (this is erotica, after all) and just varied who was doing the catching and who was being caught together. I think I've got about a dozen stories that have some element of that in them, although some were written before I created that list of variants.

This particular story started as a taboo variant, where a wife catches her husband with his sister. But I wanted to spice it up a little more, to get the wife's brother involved as well, so defined two brother-sister pairs, Jack and Jan and Dave and Abby, who cross-married each other. And I made the means of discovery technological, by use of a spy cam.

This is the only story I've written so far with two narrators. Part 1 is narrated by Jack, a private investigator, as his sister Jan comes to him and asks for a web cam to catch her husband Dave, in an affair. Dave's sister Abby is Jack's wife. It includes enough flashback to explain that Jan introduced Jack to Abby and how Jan then met Dave at Jack and Abby's wedding.

Soon, Jan has her proof and invites Jack to view it with her. You've figured out who Dave's cheating partner is by now, right? And how both Jan and Jack react to the revelation? If not, stop reading, because I'm about to spoil the surprise.

Part 2 is narrated by Dave and is mostly flashback about the past of his relationship with Abby, bringing us forward to the present point where Jack and Jan walk in to confront them.

Part 3 switches back to Jack narrating, as he and Jan confront Dave and Abby. Do I really need to tell you what happens next? Actually, I did, in the pages of the book. You just need to read it to find out, right?