"Ten-year-olds Pearl and Ben, continuing their apprenticeships at Dr. Woo's Worm Hospital, discover that Dr. Woo and the sasquatch are both sick with Troll Tonsillitis, and they must travel to the Imaginary World to find the only cure: a griffin's feather"-- Provided by publisher.

Ben and his adopted parents are devoting their lives to protecting rare and fabulous creatures (like dragons), and at the moment they're trying to save the last of the winged horses, Pegasus unicus. There are three eggs, but without their mother they won't survive and the only hope is to obtain a Griffin feather--but Griffins are enemies of the dragons, and may not be willing to help.