A couple of hunters come into a park and one tells the other that the last unicorn in the world is in that park, so they can't hunt there. She talks to herself about being sure she's the last and eventually leaves the park. She's trapped by an evil witch, but let go by a pseudo-magician named Shmendrick. They must find the Red Bull to let all the other unicorns out and on their way, they're caught by bandits. After Schmendrick gets rid of the bandits, except for their cook, they proceed on to the castle at the end of the world that has the red bull.
The unicorn meets the red bull and will die unless Schmendrick turns her into a human, which makes her fade and feel empty. But it does get them into the castle where the prince falls in love with her. They find out that all the other unicorns are in a cave under the castle, kept there by the red bull, and they learn how to get to the cave. They manage that and at a disastrous moment, the other unicorns come dashing in at the top of the waves and kill the red bull and the king. They head out, as do our group, and the world is right again.
Many of the voice actors are well-known: Mia Farrow, Alan Arkin, Tammy Grimes, Angela Lansbury, for example, and the band America does almost all the songs.