It involves a white bear that offers to take the youngest child to fix a family's poor situation. They accept and the bear takes the young girl to a castle where a man slept in the same room as her at night in the dark. As such, she could not see who it was. When she was homesick she was allowed to go home with one condition: She is not allowed to stay with her mother alone. Of course, the young girl doesn't listen and takes a magical candle from her mother. When she returned to the castle, she was able to see the face of the man that has been visiting her bed at night — who was actually the bear. After a what have you done moment he gets taken away by his troll stepmother to marry a troll princess. Before leaving, he tells her that he will be at a land East of the Sun and West of the Moon.
She sets off to find him and meets a woman and her daughter on the way. This woman gives her a golden apple and lets her borrow a horse. Next, she meets a woman who gives her a golden carding comb. A third woman gives her a golden spinning wheel and tells her that she should go find the east wind who might take her to the place that she seeks. The east wind could not help her as he never blew that far so he tells her to visit the west. After facing the same scenario, she visits the south and finally the north wind. The girl then gives up all of her golden items to the princess in exchange for a night with the prince. On the first two nights, she could not wake him. Eventually the servants tell the prince about the girl and he tosses away the drink — actually sleeping potion — from the princess that night. In the end, the girl defeats the trolls (the stepmother and the princess) by washing out the tallow of one of the prince's shirts, because the prince refuses to marry someone unable to do something so simple. The story ends with all the trolls exploding. Everyone else lives happily ever after.
It's Aarne-Thompson type 425A, the search for the lost husband, a type of which there are many variants. Compare The Feather of Finist the Falcon and Pintosmalto, and for the Gender Flip Soria Moria Castle and The Blue Mountains.
The tale, with all related versions, is reckoned to be related to the tale of Eros and Psyche, as re-told in the book The Golden Ass by the author Apuleius, from the Roman era. This version is probably the Ur-Example of the story. As everyone will understand, the girl has the role of Psyche, while the prince has the role of Eros.
For a modern novel version, see East by Edith Pattou or Once Upon a Winter's Night by Dennis L. McKiernan. There is also Ice, by Sarah Beth Durst, which adds in some Inuit legends into the mix, and The Polar Bear King.
A number of famous illustrated versions of this fairy tale have been published, including by Mercer Mayer, among others. All of the versions are slightly different. Do not confuse with the Haruki Murakami book South of the Border, West of the Sun.
Tropes in East of the Sun and West of the Moon:
- Animorphism: The result of a Curse placed upon the prince by his Wicked Stepmother.
- Beast and Beauty: For a while, the story focuses on the marriage of a bear and maiden.
- Beauty = Goodness: You can tell that the trolls are evil because they're both clearly described as hideous.
- Big Fancy Castle: The bear takes his new wife to live in a great, ornate castle.
- Color-Coded for Your Convenience: In this and many of the story variations, the groom's animal form is often white. This ties into the cross-cultural concept that white animals are believed to have magical properties.
- Curse: The prince was cursed by his stepmother to spend every day as a white bear and only turn human at night.
- Curse Escape Clause: The prince will regain his human form in permanence and be free from his Wicked Stepmother if his wife will go for one full year without betraying his trust. She fails to reach that goal before spying on him, however, and off to the trolls' castle he goes.
- Disproportionate Retribution: Poor girl makes one mistake, then must trek all over Scandinavia to right it.
- Dogged Nice Guy: The groom, who is always described as treating his bride extremely well when they get to his palace — servants to tend to her every need, great food, etc. This is a strange variant in that he already has the girl; she's just repulsed by his animal appearance.
- Earn Your Happy Ending: And how! The girl has to travel all over the world, begging sages and the winds for information to help her get her husband back, before the North Wind allows her to travel with him to the ends of the earth — and only then can she start actually trying to free him.
- Four-Element Ensemble: Mercer Mayer's retelling of the story has the role of the four winds filled by the Salamander (fire), Father Forest (earth), the Great Fish of the Sea (water) and finally the North Wind (air).
- If I Can't Have You...: The groom is cursed because he won't marry another princess, who is unpleasant and often hideous.
- Involuntary Shapeshifting: The groom turns into a bear every morning and a man every night, and cannot control this.
- Nice Job Breaking It, Hero!: While it's understandable that the bride's parents would feel Squicked that she's married a bear/wolf/some other huge and intimidating animal, they often have an iron grip on that Idiot Ball when the bride herself isn't carrying it.
- The Quest: The wife has to search for her husband by going east of the sun and west of the moon. This may or may not be an allegory for finding a nonexistent place through The Power of Love.
- Textile Work Is Feminine: The final thing she had to do was wash his shirt clean.
- Villainesses Want Heroes: The troll bride, who wants to marry the prince.
- What Beautiful Eyes!: The prince nearly always has gorgeous blue eyes, yet rarely is his hair color even mentioned, which is possibly so he doesn't outshine his wife in the looks department.
- Wicked Stepmother: The prince's stepmother — who is a troll, no less — cursed him into his ursine form, keeps him prisoner and tries to make him marry her hideous daughter.
- Youngest Child Wins: The bride is the youngest child in a very large and poor family.