Winnie's Dad decides to take her and her brother, Jerry, camping late in the season, so there are no other campers in the State Park. When their constant bickering causes Winnie to storm off in anger, she accidentally saves a troll from drowning. When she and Jerry find out the trolls have no magic because their moonstones were stolen many years ago, they agree to go on a journey with two trolls to find them.

There are specific dangers along the way which require a unique characteristic of one of the four to get through, so they must all work together and believe in each other even when it doesn't quite make sense.

The main story is sprinkled with the antics of how their father is reacting to his children going off without his explicit permission, adding to the humor of the book.