Ursula the Sea Witch is very evil...and she wants to get back at King Triton for kicking her out of the palace. She decides to use Ariel to punish Triton.
Ursula tells Ariel she can turn her into a human but that she must get Eric to kiss her....a kiss of true love.... within 3 days. If she fails she becomes Ursula's property to do with as she wishes. But Ariel must pay for this chance to win Eric's love with her voice!
Ariel decides to take the chance and signs away her voice to Ursula.