The enchanted castle is a country estate in the West Country of England, as seen through the eyes of three children, Gerald, James and Kathleen, who discover it while exploring during the school holidays. The lake, groves and marble statues, with white towers and turrets in the distance, make a fairy-tale setting, and then in the middle of the maze in the rose garden they find a sleeping fairy-tale princess.

The "princess" tells them that the castle is full of magic, and they almost believe her. She shows them the treasures of the castle, including a ring she says is a ring of invisibility, but when it actually turns her invisible she panics and admits that she is the housekeeper's niece, Mabel, and was just play-acting.

The children soon discover that the ring has other magical powers...