Wanting desperately to achieve the same rank as the twelve generations of scouts in his family before him, Samuel Moss discovers that he will have to earn the Moon Frog Badge in order to complete his mission and so goes on a wild adventure to find the rare creature who only appears from hibernation on a special night once every thirty years.

Accompanied by his aunt's Norwegian elkhound, Ibsen, twelve-year-old Samuel ventures into a weird forest filled with strange and dangerous creatures to rescue his younger sister, Martha, who has been mute since their parents' recent death.

When an attempt to steal Shirley, a rare sea-unicorn, results in the sinking of their houseboat, ten-year-old Thora, her mermaid mother, and Mr. Walters stay at the estate of old friends and find a solution to the friends' financial troubles, but the would-be thief is still on their trail.