Ellabeth discovers a lost and lonely weather sprite, Lumi, who has been left behind by his master, the wizard Kai Laing. The Unicorn Riders must reunite the sprite with its master. But first they must find the sprite's missing elements - earth, air, fire and water - before the weather in Avamay spins out of control. Lord Valerian wants the sprite for his magical creature collection and sends an army to pursue them. Can Ellabeth and the Riders find the elements in time? Can Ellabeth protect Lumi from Lord Valerian?