Once upon a time -- there were two brothers, Gerald and Jimmy, and a sister, Kathleen, who could not go home from boarding school because of measles, and after some wangling, they ended up staying at the sister's school, where there is a single teacher, the French mistress, staying during the holidays. Gerald is polite and persuades her it would be nicest all around if they went out by day with their meal in a picnic basket, and they set out in search of a rumored cave.
They find it. It leads to a garden, with a castle over it, and in the garden's maze, they find a girl asleep, as in Sleeping Beauty, Awoken, she assures them she's a princess, takes them inside, and shows them treasures, including a ring that turns people invisible.
It turns her invisible.
At which point, contrite, she confesses to merely being the housekeeper's niece Mabel, and having just been playing when she said it could make her invisible. It does not restore her to visibility.
The effect wears off in time, but that's only the first of their adventures revolving about the castle and the ring. It includes performing Beauty and the Beast, statues that come to life, why the lord doesn't live in the castle, talk about getting a good hotel, a rivalry between two businessmen, dealing with burglars and much more.