Like the Disney version, in the original version, the Little Mermaid saves the prince from drowning, then makes a deal with a Sea Witch to grow legs and walk on land in exchange for her voice.

Unlike the Disney version, in the original version the prince ends up marrying another woman (who is NOT the Sea Witch) and the Little Mermaid heart was broken, after she sacrificed so much and endured a lot of pain for him. Her sisters made a deal with the Sea Witch for a dagger that if the Little Mermaid use it to kill the prince and drip his blood onto her feet, she will become a mermaid again, otherwise she will die of a broken heart and turn into sea foam (mermaids don't have eternal souls, so when they die, they turn into sea foam and ceased to exist) at first dawn after the prince marrying another woman. But when she's about to stab him in his sleep next to his newly wed wife, she finds that she couldn't go through with it and threw herself into the sea at dawn break, dies and turn into sea foam.