This original fairy tale was written by Hans Christian Andersen, and it's vastly different from its Disney sister. Here's what goes down: the little mermaid spots her prince aboard a ship and rescues him from drowning after a terrible storm breaks the vessel. From there, she becomes obsessed with him and can't stop thinking about him, so much so that she sacrifices her voices to get a pair of legs from the sea witch. That all checks out with the animated movie, but then it takes a sharp left.
There are different conditions for this leg-bestowing spell. If the mermaid can't get the prince to marry her, she'll die and turn into sea foam. Additionally, having legs creates torturous pain for the mermaid. Every step feels like knives, and she even bleeds when she takes steps. Also, as it turns out, the prince is in love with someone else entirely: a young maiden who found him on the beach just after his rescue. He eventually marries this maiden instead. As a last-ditch effort to prevent their sister from dying, the other five mermaid princesses under the sea give their hair to the sea witch and obtain a knife. If the little mermaid drives it into the prince's heart, she will get her fins back.
The little mermaid opts not to kill the prince and throws the knife into the sea. She falls into the waves, but instead of becoming sea foam, she finds herself "among the daughters of the air" to live for the rest of eternity.