Daffy Fernald loses her brother's toy rocket when it flies into a storm drain. She knows Alvin will be upset if he finds out, so she climbs into the dark tunnel to search for it. She spots a stranger, Ernie Nelson, and sees that he's carying a gun. She races home and tells Alvin, but the would-be inventor refuses to believe her.

A 13-year-old brainiac invents an ingenious listening device, which he uses to bring a crooked city official and his pals to justice