19 year old Nick has mixed feelings about his mother. On one hand, she's his mom, the woman who raised him, but she’s also the hottest woman he has ever seen, and Nick can't stop thinking of her in that way.
Those thoughts started several months ago when Nick discovered videos of his parents having sex, and has been masturbating to them ever since.
When Nick was snooping on his father's computer, looking for more videos, he finds one of him with another woman. Nick tells his mother and, enraged, she throws his father out and swears she'll get even.
The next night, Mom calls Nick into her bedroom and tells him she's found out he's been watching her videos. Nick's embarrassed and tries to apologize, but Mom tells him not to worry about it.
The way she sees it, she owes Nick's father some payback, and seeing as how Nick has been fantasizing about her, what better revenge to take than to have sex with his son?