The story is about a unicorn who overhears two hunters one day, and discovers that she is the only unicorn left in the world. She cannot believe the news, and is from then on determined to find the rest of her kind. Along the way she meets a man called Schmendrick , a wizard whos magic only works 50% of the time, and a woman called Molly Grue. They join her quest, to find the other unicorns.
The Unicorn hears from a butterfly about a red bull, who chased all the unicorns away. Together, the three of them make there way to the castle of King Haggard , where the red bull supposedly is. When they are attacked by the Red Bull along the way, Schmendric turns the Unicorn into a human girl, so the Red Bull doesn't see her. They continue to the castle, and meet King Haggard and his son, Prince Lir. Schmendric makes up the name Amalthea for the Unicorn. The Prince, not knowing what Amalthea was, falls in love with her. Becoming less unicorn and more human, Amalthea starts to return his love, and forgets she was ever a unicorn.
When Schmendric finds the Red Bull's lair below the castle, it confronts Amalthea again. She turns back into a unicorn and tries to escape the Red Bull, which tries to drive her into the sea, where all the other unicorns are trapped. Lir dies trying to protect her.
Seeing Lir die for her, gives her courage and she turns on the Red Bull, driving it into the sea, setting all the other unicorns free. She then brings Lir back to life with her horn and says farewell to Molly and Schmendric. She rejoins her own kind, but being human has changed her life forever.