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