Fonzie's going out of town for a week to the big demolition derby, so he asks Richie to watch his girl, Denise, for the week and make sure nobody's dumb enough to move in on "Fonzie's girl."
Later that day at Arnold's, Richie asks Wendy (the waitress) to send Denise a soda from him, after explaining that he's just watching her for Fonzie. Wendy delivers the soda to Denise and tells her why. Denise is upset that Fonzie "acts like she's his property," even though they've only been going steady for a week. She decides to show him a thing or two, and asks Richie if she can come over to his house that night. After some hilarious dialogue about the two from Mr. & Mrs. C., Denise cons Richie into taking her to Inspiration Point, where she seduces him into making out with her. Later in the week, he asks Denise's friend where she is at school, who reveals she has caught mono. Richie then reveals to Potsie and Ralph that he has been to Inspiration Point with Denise every night that week, and begins to exhibit several symptoms common to mono. He goes to get a blood test at the college free clinic and then goes back to his house and takes everything that might have his germs on them. Later, he reveals to Fonzie what happened. The ending is kind of a cop-out, but still funny nonetheless, capitalized by Ralph begging the Fonz to belt him one in the face.
Not a standout episode of the series, but very funny and memorable for me.