He discovered his married sister having an affair with his best friend. Instead of being shamed or apologetic, she asked him for help finding someone who could impregnate her.
Watching TV with her husband a documentary about "Dogging" came on and much to her surprise she felt compelled to watch it fully, but a while after she became curious about the subject and wanted to know more.
A private investigator is hired to investigate some financial trickery. An assistant, a forensic accountant, accompanies him, and despite her initial holier-than-thou attitude, her own marriage, in shambles, drives her into the arms of the private dick. It's a torrid affair, her doing sexual things with and learning sexual things from this guy as she learns what it is like not to be ignored.
As we go through life, most of us will come across our own personal Nemesis at least once in our lifetime. Peter had it figured that his Nemesis had always then there, bugging him, ever since they'd been children together.