Not every story in children's literature has a happy ending.
Written in 1836 by Hans Christian Anderson, The Little Mermaid is a far cry from the watered down version presented by Disney in their animated movie.
The Little Mermaid is the youngest of the sea king's children and is very easy on the eyes. The King allows the daughters to go up to the surface on their fifteenth birthday almost as a right of passage. Once it is the Little Mermaid turns fifteen she gets to go to the surface just like all of her sisters before her. When she goes up she spies a prince on his ship celebrating. A storm hits and the prince is injured and thrown from the ship. So the little mermaid saves him and brings him to the shore where he is found by a group of girls from the local monastery. The Little Mermaid becomes obsessed with this prince and decides to go to the sea witch for help. The sea witch trades the Little Mermaid a potion that will give her legs for her voice.
This is where the similarities between Disney's Little Mermaid and Hans' Little Mermaid end. From this point on Hans' version is much darker than the other version. According to the original version the Little Mermaid goes to the shore where she last left the prince, drinks the potion and goes through unbelievable pain to have her tail split into two legs. Also according to the story the Little Mermaid was told that if the prince could not love her enough to forget his parents for her sake then she would die but mermaids do not enjoy an after life like that of humans, they become foam on the top of the oceans. It's a trade off for their ability to live for 300 years.
The prince finds the Little Mermaid and takes her to his castle. There he becomes very fond of the little mermaid. He loves her but not enough to forget about his parents for her sake. For he truly loved the girl who saved him when he washed ashore but they could not be together because of her studies at the monastery but, he told the Little Mermaid, that if he could not find that girl he would marry her.
The prince finds out that he must go and visit a nearby Kingdom at the request of his parents. While on the ship at sea the Little Mermaid is visited by her sisters and is told how unhappy their family is with out her. They arrive at the neighboring kingdom and to the prince's great surprise the princess that he is to marry is the very girl who he fell in love with when he washed on shore that day. The Little Mermaid stands by and watches him marry this other woman.
That evening they are back on the ship sailing home when the Little Mermaid's sisters appear at the surface, completely bald. They give her a knife and tell her that if she kills the prince with that knife his blood will give her back her fins and she could come back. And the reason as to why they are bald? They traded their hair for the knife.
The Little Mermaid goes to where the newlyweds are sleeping and is about to strike the heart of the prince. But she can't. She loves him too much to hurt him even if he doesn't love her back. So she flings both herself and the knife into the ocean. Instead of dieing she realizes she is being lifted up. The daughters of the air rescued her. They made the Little Mermaid one of them so that she could earn an eternal soul through the protection of children.
