The day starts out fine. It's Mr. Wilson's birthday and guess who shows up uninvited? Dennis Mitchell and a few of his bug friends. After getting a cake in the face and falling down the stairs in a wagon, Mr. Wilson wishes Dennis had another old man to bother. His prayers are answered when Dennis' grandfather, Mr. Johnson, comes over announcing he's going to move in with them. So once again all is fine until Mr. Wilson finds out he's getting old, and Mr. Johnson can do everything better than him. One day at a carnival, Mr. Wilson meets the Professor and his assistant, Sylvester, who claim to be specialists in making people younger--for a price. It's a scam but Mr. Wilson is too naive to know. The first few attempts at decreasing Mr. Wilson's age fail thanks to Dennis. Mr. Wilson tries to do things better than Mr. Johnson when he follows them to the swimming pool but fails. But it's Dennis who catches the swindlers at their own game when Mr. Wilson wants to sell his house and the Professor and Sylvester show up disguised as plumbers, electricians, realty men, and exterminators and make large holes in Mr. Wilson's house--not to mention his check book, but it's Dennis to the rescue and everyone is happy. Sometime later, Mr. Johnson takes Dennis to the Grand Canyon in his motor home. The mistake is that it is parked awful close to the canyon and its wheels are stopped with a rock, which Dennis collects while rock hunting. Back at home, Mr. and Mrs. Wilson watch Mr. Johnson getting rescued from the canyon and Mr. Wilson proclaims, "He's a menace!"
