The realm of magic is being threatened by the realm of logic, so Carolinus, the green wizard decides to shield it for all time. Ommedon, the evil red wizard, stands in his way. Carolinus then calls for a quest that is to be led by a man named Peter Dickinson, who is the first man of both the realms of science and magic. It is Peters job to defeat Ommedon.

From a riddle-speaking butterfly, a unicorn learns that she is supposedly the last of her kind, all the others having been herded away by the Red Bull. The unicorn sets out to discover the truth behind the butterfly’s words. She is eventually joined on her quest by Schmendrick, a second-rate magician, and Molly Grue, a now middle-aged woman who dreamed all her life of seeing a unicorn. Their journey leads them far from home, all the way to the castle of King Haggard.