Mark and Kara are twins. The two have just turned eighteen. The two decided for their birthday that they'd see Madam Hex. Mark and Kara were in love with each other. Nothing Hex hasn't dealt with before. First she spoke to them one on one the together.
Hex decided to speak up. "After visiting with you, it is rare to get such polite young folks. And two who give me a chance. Thanks to those fakes, the real ones get visits, but...."
"Nasty attitude?" Mark asked.
"Exactly. However, your case isn't the first I've crossed. You are brother and sister yet you love sex with each other. No it isn't new but I tell you. Unless you change, critize, doubt, or whatever you want, but unless you two stop you will become animals. Some say it's a curse, others a gift. A few have said it's a test. But few see it as a sign to stop. Here's how it works. Each time you climax, an animal part becomes permanant. No going back. You both, when done which ever one has a pussy will be in heat. The longer you fight the stronger the lust gets. Masterbation doesn't change unless incestial thoughts are in your head. Now if you two don't mind a little boy is in line. Warning. The first is always permanant so don't panic."
After that the two parted. Not sure if to believe the two go home. Mark and Kara kiss and hit the showers, each discovering the ears of.....