Marcia comes home from school smiling and looking dream. She tells Jan that she is love with Andrew Whittaker, and makes Jan swear not to tell. But after they leave, Peter comes in and learns about Marcia's love from a tape recorder he had planted in the bedroom. When he starts quoting snippets of the secret conversation to Marcia, she assumes Jan has broken her promise and starts arguing with her. Carol and Alice are concerned and puzzled at the shouting they heard.
Peter comes in on Greg while he is fixing a clock. Marcia comes in to see Greg and Peter is ordered to leave. He does, but he plants the tape recorder on the way out. Greg then entrusts Marcia with a secret. He has a problem that he does not want their parents to find out about; he forgot to return a library book and it is 40 weeks overdue. The fine left him broke and the parents may not advance his allowance as this is his third offense. He asks to borrow some money from Marcia. Later, he drops hints about the secret conversation to Greg that has Greg assuming that Marcia blabbed, and this leads to another argument. Later, Peter pulls the same trick on Cindy and Bobby and they start arguing as well.
The parents and Alice are puzzled over the arguing and do not think it is coincidence. At dinner, the kids eat in cold silence and Mike demands to know why. This has the arguments erupting again. Then Mike and Carol notice how smug Peter is, get suspicious, and ask for explanations. Peter confesses what he did and only meant it as a joke. The parents are not impressed, but as it is a first offense, they settle for Peter apologizing to his siblings. He does, and to all appearances everything is forgiven.
But Marcia and Greg feel Peter got off too lightly with an apology and think he will just do it again. They decide to give him a taste of his own medicine by planting a conversation on the tape recorder that has Peter thinking the parents are planning a surprise party for him with lots of presents for bringing home an A instead of going off on their upcoming skiing weekend. Greg then gives the recorder to Peter, accusing him of being up to his tricks again. Peter is indignant at the accusation, but of course he listens to the recording. Greg is listening at the door and is gleeful that Peter has fallen for the bait. Peter starts acting like he knows nothing, becomes suprisingly extra-helpful, and drops hints about what presents he would like. Alice, Mike, and Carol are puzzled while Marcia and Greg giggle.
Mike's typewriter dies and he brings out the tape recorder to record his notes on it instead. He and Carol discover the recording Greg and Marcia made.
The parents go off on their ski trip, but Peter has been led to believe they will secretly come back to throw the party. He starts dressing up for the party. He comes downstairs to find no party and he is puzzled. He looks everywhere for his party, but it is nowhere to be seen. When Marcia and Greg see Peter dressed up, they are struck with remorse and decide to confess. Peter is crushed, but understands why they did it.
Then Mike and Carol return, laden with presents to throw the surprise party Peter got for scoring an A. They then have a quiet word with Marcia and Greg.
A tape recorder is included in Peter's presents. Peter tells Marcia and Greg they are free to borrow it, but also says that there is material on the recorder, and he does not want them to interfere with - a hint that he wants them to listen to it. Naturally, they listen to it after he leaves. They hear Peter saying, "I, Peter Brady, do solemnly swear to forgive Greg and Marcia for the crummy thing they did to me. And shame on you for listening when you shouldn't have." The recording ends with a giggle, and Marcia and Greg start laughing as well.