Classicly

The Golden Key is a fairy tale written by George MacDonald. It was published in Dealings with the Fairies. It is particularly noted for the intensity of the suggestive imagery, which implies a spiritual meaning to the story without providing a transparent allegory for the events in it.

A young boy listens to his Great-aunt's stories about a magical golden key found at the end of a rainbow. One day, he sees an immense rainbow and sets out to find its end in an enchanted forest. As the forest is in Fairyland where everything has an opposite effect, the rainbow only glows brighter when the sun sets. He finds the key, then it dawns on him that he does not know where the lock is.