Syd, who lives with her boyfriend James, goes to complain to her neighbor about the leak in the ceiling. Her neigbor is photographer Lucy Berliner and Syd starts to fall in love with her.

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...