The king has his hands full with 12 naughty daughters. Every night through a secret portal in their room they sneak off to dance the night away with handsome princes. The king tries desperately to figure out where they go but to no avail. He then sends a decree to all men in his kingdom that whoever could figure out his daughter's secret in 3 days could pick on to be his wife.
A soldier returning to the kingdom passes by an old woman's house and tells her that he is on his way to see if he can solve the princesses' mystery. She gives him a cloak that will make him invisible and warns him not to drink anything the girls give him. When he enters the princesses' room they offer him drink but he secretly pours it into a sponge under his chin. He is able to follow them for all three nights into the secret world and collects leaves of gold silver and diamonds for proof to the king of his findings.
When he reveals to the king of his daughter's secret and shows the king his findings the king offers to the soldier any of the daughters to be his wife. He chooses the eldest and she accepts seeing he was no fool.
I really enjoy this story, especially more of The Brothers Grimm's version. There is a Barbie movie version of it and while cute I really like the original version better.