Gerald, Kathleen, and Jimmy are siblings anxious to go home on vacation from their boarding school, but when measles strikes a cousin staying at their parent's house, they must stay at the school under the supervision of a distracted French governess.

Almost immediately, the children decide to go exploring, and soon stumble upon a hidden castle-like mansion, where they find an elaborate sculpture garden, a hedge maze, and a sleeping princess who gives them a tour of the castle and shows them a room of magical wonders. 

While boasting about the powers of a magic ring, their guide turns herself invisible...much to her own surprise! The “princess” turns out to be Mabel Prowse, the niece of the housekeeper, who is surprised as anyone that the ring contains real magic—and more surprised when she learns the ring won't come off her hand!

The children must learn the secrets of the ring and about the true nature of wishes: sometimes the thing we most want is not at all the thing that we need.