A whimsical children's fantasy follows a widow and her twins, whose hair turns blue and green after their chemist father's experiments, as they encounter a mysterious visitor and a self-playing enchanted banjo. The family discovers secrets hidden in the father's cupboard and embarks on extraordinary voyages by air and by a submarine express to a remote island ruled by a playful monarch. There they meet fairy creatures and royal twins, encounter magical devices such as a fairy telephone and a golden ladder, visit candy- and gold-filled realms, confront a wicked fairy, and ultimately resolve the royal troubles before returning home.