When Ursula first appears in the film, she states through monologue that she once lived in the royal palace of King Triton, the king of the underwater city of Atlantica. She was banished from Atlantica and made her trang chủ in the remains of a leviathan where she waits the chance to get her revenge on Triton and become Queen of Atlantica.

The entrance to Ursula's leviathan trang chủ is lined with a garden of writhing polyps which were merfolk who had been previously gone to Ursula for help, but found themselves unable to fulfill their side of the bargain, leading Ursula to claim them as her own and added them to her collection. The garden of polyps are part of the original fairytale, but are simply guards of the way to the sea witch's home, and not condemned merfolk.

Ursula commands her minions, moray eels Flotsam and Jetsam, to watch Triton's youngest daughter Ariel, whom might be "the key to Triton's undoing". Opportunity arises when Ursula learns that Ariel has fallen in tình yêu with a human, against Triton's explicit rules. Flotsam and Jetsam lure Ariel to Ursula's lair, claiming that only Ursula can help her.

Through the song Poor Unfortunate Souls, Ursula proposes an agreement where she will transform Ariel into a human for three days, during which time, she must recieve the "kiss of true love" from Eric. If Ariel succeeds, her transformation into a human will be permenant, but if she fails, she will turn back into a mermaid and be bound to Ursula for eternity. The price for the transformation is Ariel's voice. Ariel agrees and signs a magical contract, so her voice is magically pulled into Ursula's Nautilus shell chuỗi hạt, chuỗi hạt cườm causing Ursula to laugh gleefully as her plans begin to unfold. Ariel's tail is chẻ, phân chia, split into legs. In comparison, the original fairytale involves the sea witch taking the little mermaid's voice bởi cutting off her tongue, and her tail was transformed into legs bởi a magic potion which the mermaid was to drink on the bờ biển, bãi biển surface.

When Ariel comes "too close" to succeeding in her task, Ursula takes the form of a beautiful human female with Ariel's voice in order to sabotage her relationship with Eric. Ursula adopts the name of "Vanessa", and when in this form, her voice is provided bởi Jodi Benson, who also voices Ariel. The concept of the beautiful dark-haired rival for the prince's affections is another plot point adapted and modified from the original fairytale.

As Vanessa, Ursula bewitches Eric to marry her, but the wedding is disrupted and the Nautilus shell containing Ariel's voice is broken, breaking the enchantment and returning Ariel's voice back to its owner. However, the sun sets before Ariel and Eric can Kiss and Ariel changes back into a mermaid. Ursula, restored to her normal form, grabs Ariel and jumps back into the sea, where she is confronted bởi King Triton. She reveals her true goal, and forces Triton to choose between his freedom and his daughter's. Triton agrees to take Ariel's place, and Ursula takes the crown and magical trident, making her the Queen of the Seven Seas. Ursula grows large and creates a storm and whirlpool. The Sea Witch then imprisons Ariel at the bottom of the whirlpool, and decides to test her new powers bởi killing the princess. Ursula fires multiple destructive blasts at Ariel, and the princess just barely manages to avoid them. Ursula laughs and prepares an unavoidible blast that would destroy Ariel forever. But right when she's about to fatally blast Ariel, Eric impales Ursula through her belly, and the powers of the trident backfire on her. When Ursula was defeated, all the merfolk that had been under her power, including Triton, are released from their bonds.

In the 2006 Platinum DVD, deleted scenes revealed early concept ideas where Ursula is Triton's sister, making her Ariel's aunt. This concept was not used in the final film.