Marcia is admiring herself in a pair of earrings she borrowed from Carol. When Jan comes to tell Marcia that a boy is phoning for her, she puts the earrings away in her drawer. Cindy asks if she can try them, but Marcia says not to touch them as they are borrowed from their mother. But of course touching them is precisely what Cindy does once Marcia and Jan leave the room. She is admiring herself in the earrings in the bathroom mirror when Carol knocks at the door because she has a new blouse for her. Cindy quickly hides the earrings in a towel on the bathroom sink. When she goes back to the bathroom, she finds the earrings are no longer in the towel. They have disappeared somehow, and now she is in touble.
Peter is trying out his new Sherlock Holmes kit. Cindy comes to him in the hope that he and the kit can help her. Peter is happy to take on his first case, which he dubs "The Great Earring Caper." Cindy will be his assistant, just as Watson was to Holmes. Peter deduces the earrings slipped out of the towel and down the drain. He gets Mike's tool kit to open up the drain, with no results. He now deduces that the earrings were removed by human hands. They go around the house to collect fingerprints (causing some raised eyebrows along the way). But of course everyone's fingerprints are on the bathroom sink. Peter goes to Plan B - which he has to look up in his detective manual.
Meanwhile, Carol and Mike are planning to go to a costume party. Possibilities that have been discussed are Romeo and Juliet, Napoleon and Josephine, Gertrude and Claude (imaginary couple as Mike considers the first two unoriginal), George and Martha Washington, and Adam and Eve, but eventually they settle on Antony and Cleopatra. This choice means that Carol wants the earrings for the party, which puts more pressure on Cindy and Peter to find them.
Peter has been boning up on his detective book and decides the next course of action is to question everybody in order to find the person most likely to have taken them. The questioning is not very professional and raises more eyebrows, but it eliminates suspects. The time comes for Carol and Mike to get ready for the party. Peter's investigation has gotten nowhere and he has lost confidence in his detective abilities. Cindy has no choice but to tell Marcia. Marcia drags Cindy before Carol and Mike to explain what happened.
Mike takes over the investigation and reconstructs the path of the earrings. After Cindy left the bathroom, Alice came in to collect the laundry. She put the towel in the laundry bag, and inadvertently tipped the earrings into it. Then Alice put the towel back because of Carol's instructions about teaching the kids to clean up after themselves. Alice left the laundry bag in the hall while she went to get something. Jan found the bag and took it to the service porch. Finally, Carol found the bag and emptied the contents into the washing machine. They check the washing machine, which yields one earring and part of the other one. The Great Earring Caper is solved.
Cindy is sorry and promises her mother she will never again touch anything she is not supposed to. Carol appreciates it, but warns Cindy they are going to have a long talk the next day. Right now, she and Mike have to go to the party. Cindy asks Peter if she can borrow one of the disguises in his detective kit to hide from Carol.
Carol and Mike return home from the party. They won third place, and the Cunninghams came first. They tell Alice she will never guess who the Cunninghams came as. Romeo and Juliet? Nope. George and Martha Washington? Nope. The Cunninghams came as Sherlock Holmes and Dr. Watson.