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Potsie tosses a football to Richie as they come out of school, and Fonzie catches it. Fonzie plans on going to Rockford, Illinois, for a demolition derby championship (representing Happy's garage) for a week and wants Richie to watch his girl, Denise Hudson. Richie agrees. Richie sends Denise a soda at Arnold's, and Wendy delivers it to her. When she hears about what Fonzie has Richie doing, Denise plans to test Richie's trustworthiness. Denise wants to come over to watch some television at Richie's that night. Howard looks at a TV Guide to decide what they will watch. Denise starts hugging Richie, and Marion and Howard go into the kitchen to leave them alone. Joanie continues to sit on the floor and stare at Richie and Denise, so Richie sends her to the kitchen. Joanie walks into the kitchen and finds Howard and Marion kissing. Marion brings some animal crackers into the living room, and Richie and Denise leave for Inspiration Point. Denise continues to entice Richie, but Richie insists that she is Fonzie's girl. He finally gives in and makes out with Denise. At Jefferson High, Denise's friend, Joyce, informs the guys that Denise has mononucleosis. Richie fixes the basketball net at the house while he tells Potsie and Ralph that he has been up at Inspiration Point every night that week with Denise. The guys believe Richie has the symptoms of mononucleosis. Howard fixes the vacuum cleaner, and Richie tries not to breath near him. Richie drinks Joanie's lemonade and prevents Howard from drinking from it by saying it's his favorite glass. Upstairs, Richie reveals to Howard that he might have mono. Howard suggests that Richie tell Fonzie about what happened with Denise. Fonzie returns after a week and goes into his office at Arnold's (the restroom) to talk to Richie. Richie tells Fonzie everything. Fonzie thinks about hitting Richie with an exhaust pipe but finds a loophole in the "rules of the game" in that he had been with a girl on his trip before Richie was with Denise. Richie gets a black eye when a kid opens the bathroom door in his face. Richie says he was hit by the door, but everyone thinks that it was Fonzie that hit him. At the house, the family plays a card game.

Songs in this episode:
"Que Sera, Sera (Whatever Will Be, Will Be)" by Doris Day (1956)
replacement instrumental
"Who's Sorry Now" by Connie Francis (1958)
"Cry" by Johnnie Ray (1951)