In order to protect her from the Red Bull, who had driven all the other unicorns into the sea, the wizard Schmendrick transforms her into a human shortly before they reach King Haggard. The Unicorn spends too much time in human form, falls in love with the Prince, and does not wish to return to her original shape. However, Prince Lir understands that the other unicorns must be rescued, and they accompany Molly Grue and Schmendrick into the caverns beneath King Haggard's castle, and there the Red Bull reappears. There, Schmendrick returns her to her true form, and the Bull chases her out of the caves and begins to force her toward the sea. Lir attempts to protect her, only to be easily and brutally dispatched by the Bull.
However, his sacrifice galvanizes her; she fights the bull and forces him into the sea instead, as all the unicorns in the world come crashing out of the waves.
When it's all over, the Unicorn restores Lir, although she leaves before he awakens. She also speaks with Schmendrick, who with restoring her to her true shape finally attained true control of magic. The experience has changed her and she knows regret, which no unicorn should be able to feel. However, she accepts this, thanking the wizard for the experience, and begins the journey home.