Mary Ellen Rogers has gone away to school (some fancy boarding school, I assume) and is back for a short time and having a big dance at the country club. She invites Wally, who doesn't want to go, but can't say "no" on the telephone.
Wally's pals are visiting him the next day, shooting the bull out in the garage, and they know all about the invitation. Lumpy asks Wally what his "secret" is, since Mary Ellen (Pamela Baird) now is the hottest girl around. They - Lumpy, Chester and Tooey - are impressed Wally is going out with her. While they are visiting, Mary Ellen comes to see him and is waiting in the living room. All of them go inside. When asks Tooey what he's doing, he replies, "We all came in to watch Wally talk to Mary Ellen." They stand there and gape at her (great expressions, particularly on Tooey) while Wally introduces them.
Mary Ellen coyly remarks she came over to show Wally the dress she's wearing at the dance. Now the boys' (not Wally) eyes are popping out of their heads as Mary Ellen dazzles them. Mary Ellen finally gets Wally alone outside to tell him she entered the two of them in a "cha cha contest" for the dance. "I just know you're a smooth dancer by the way you walk," she purrs at him. (Boy, I don't remember girls being aggressive like this back in the late '50s!).
Poor Wally now has to learn the cha-cha-cha, but doesn't want anyone to see him, of course. He bars the door to the bedroom while he practices to an instructional record. Beaver's listening at the door, making weird faces. He then goes around the outside of the house and looks into the window. Wally catches him and tells Beaver he better keep quiet. Beaver responds, "Gee, Wally I wouldn't say anything. I would be too ashamed to let anybody know I had a brother who danced by himself."
Anyway, not much eventful happens in the rest of the episode. The lesson in here - told to Wally by his dad, who was nice enough to pay for a last-minute dancing lesson for him, is that one would be better off telling someone he's doesn't know how to do something rather than pretending he does and then trying to learn it at the last minute.