Mike and Peggy think they might be expecting again, but later Peggy is diagnosed with having a cyst instead. The experience launches them into a debate about whether or not they should have another child. Meanwhile, the kids spy on Mike and Peggy to try to figure out what all their unusual behavior is about. When Pat overhears that they are going to have a little sister, the Cleary kids decide to dig deeper and continue their investigation, which only leads them on a downward spiral.

After seeing how loving Pat's friend's parents are, the Cleary boys beg Peggy to tell them she loves them. Playing on their insecurities, Peggy posts a ranked list of their names on the fridge in the hopes that it will bring out their competitive instincts, and someone will divulge who broke the dining room light fixture. Charged with several emotions, Timmy incriminates his brother but feels so guilty that he gets a horrible stomachache and heads to the hospital, where he unexpectedly reveals another secret. Back at home, Mike invites Grover, a co-worker, disguised as an amateur detective to their home to help compel a confession out of the guilty Cleary child.