Three children discover an old country estate during their school holidays. When they're exploring it they come upon a mysterious young girl claiming to be a fairy princess. She shows them the castle's treasure, including a ring that will turn you invisible. But no one is more surprised than she when she slips it on her finger ... and disappears! The ring is indeed magical, and the children waste no time finding out what else it can do.

An invisible princess, a magic ring, and more adventures than you could dream of. This is what Gerald, Kathleen and Jimmy find when they stumble upon a mysterious castle. At first it all appears to be a lark. But the children soon discover they need all their bravery and ingenuity to contend with the castle's supernatural forces.

An invisible princess, a magic ring, and more adventures than you could dream of. This is what Gerald, Kathleen and Jimmy find when they stumble upon a mysterious castle. At first it all appears to be a lark. But the children soon discover they need all their bravery and ingenuity to contend with the castle's supernatural forces.