The Whiz Kid and the Carnival Caper (1976)

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 carrying a gun. She races home and tells Alvin, but the would-be inventor refuses to believe her.