In this fantasy adventure, four children — Penny, the leader; Pamela, her common-sense sister; Peter, whose life’s ambition is to become a garbageman; and Patsy, who collects frogs in her pockets — set out in search of their baby brother, Paul, better known as “The Pollywog,” who has vanished mysteriously from their playhouse. Accompanied by their fearless pets, the children descend through a secret trapdoor into a strange underground world of mushrooms, whose green inhabitants know only one word: “OG!”