After Lisa is diagnosed with complications due to Middle Child Syndrome, the Simpsons decide to adopt another child to balance things out. That child turns out to be Nelson Muntz, and it's not long before he becomes head of the household, and Homer starts to suffer from Displaced Dad Syndrome. Then, Pieman and the Cupcake Kid encounter another uncouth and undercooked evil genius in the guise of Professor Frink, whose zeal to fund his research has some unexpected and embarrassing effects on the pastry-themed pair. Finally, a science experiment run amok leads to a population explosion of Barts!