Mahoney plays babysitter for the day, when asked to take care of the new neighbor's baby, Nellie. What he doesn't know is that high-tech criminals are trying to steal top-secret transistors hidden in Nellie's rattle, before Aunt Mona delivers them to the government. But when baby Nellie wanders out of the building with the rattle, a wild goose chase involving both crooks and cops ensues throughout the city. One must wonder, which poses a bigger challenge for the P.A. grads; nailing bad guys, or changing diapers?