Plot
A female unicorn learns from two hunters and a butterfly that she is the last of her kind since a malevolent entity called the Red Bull has herded nearly all of the unicorns to the ends of the earth. The Unicorn then sets off on a journey to find them.
The Unicorn is captured by the witch Mommy Fortuna and displayed in her Midnight Carnival. Most of the attractions are normal animals enhanced by illusions to appear as mythical beasts. Fortuna uses a spell to create another horn on the unicorn's head, as the carnival visitors cannot see her real form. Fortuna keeps the immortal harpy Celaeno captive as well, deeming the risk secondary to the deed's prestige. The unicorn is befriended by Schmendrick,[13] an incompetent magician in the service of Mommy Fortuna. With the help of Schmendrick, the Unicorn escapes, in the process freeing the other animals including Celaeno, who kills Fortuna. The Unicorn and Schmendrick continue traveling until Schmendrick is captured by a group of bandits led by Captain Cully, who ties him to a tree. While attempting to free himself, Schmendrick brings the tree to life, and it attempts to seduce him until the unicorn arrives and frees him. Molly Grue, Cully's lover, weeps at the sight of the unicorn and abandons Cully to travel with them.
When nearing the castle of King Haggard, the group encounters the Red Bull, a monstrous fire elemental. It attacks the unicorn, and Schmendrick transforms her into a woman to save her. The Bull loses interest in her and departs, but the Unicorn panics over her newfound mortality. Schmendrick promises to undo the transformation upon completing the quest.
Schmendrick, Molly, and the now-human Unicorn proceed to the castle. Haggard is at first unwelcoming. Schmendrick introduces the Unicorn as Lady Amalthea, and requests that they become members of Haggard's court, only to be told that the only occupants of the castle are Haggard, his adopted son Prince Lír and four ancient men-at-arms. Haggard agrees to lodge the trio, replacing his more competent wizard, Mabruk, with Schmendrick, and setting Molly to work in his scullery. Mabruk leaves after recognizing "Amalthea" for what she is, jeering that by allowing her into his castle Haggard has invited his doom. Amalthea begins losing her memories and falls in love with Prince Lír, and considers abandoning her quest in favor of mortal love. Haggard reveals to Amalthea that he ordered the Bull to round up the world's unicorns and imprison them in the sea by his castle so that Haggard can admire them every day.
The castle's cat divulges the location of the Bull's lair to Molly. Molly, Schmendrick, and Amalthea, and Lír enter the bull's den and are trapped there by Haggard. Schmendrick reveals Amalthea's true identity to Lír, who declares that he still loves her. Amalthea contemplates abandoning the quest to marry Lír, but he dissuades her. The Bull appears, no longer deceived by Amalthea's human form, and chases her. Schmendrick turns Amalthea back into a Unicorn, but she is unwilling to leave Lír's side. The Bull begins driving her toward the ocean, and Lír is killed while defending her. Enraged, the Unicorn forces the Bull into the sea. The missing hundreds of unicorns emerge from the water, causing Haggard's castle to collapse into the sea as he falls to his death.
On the beach, the Unicorn magically revives Lír before leaving him. Schmendrick assures Lír he has gained much by earning the love of a unicorn, even if he is now alone. The Unicorn later bids goodbye to Schmendrick, who feels guilty for burdening her with the experience of mortality, which could make her unable to properly rejoin her kind. She assures Schmendrick that he helped restore unicorns to the world and made her understand regret and love. Schmendrick and Molly watch the Unicorn depart for her forest home.
