Synopsis:Vanessa and her mother buy an old house from an elderly lady who warns Vanessa never to go into the attic. Vanessa chooses the attic for her bedroom, and is mysteriously transported to a magical land called Partequineus. She meets four girls who are trapped there, kept prisoner by a dragon. The girls warn her that if the dragon captures her, she will have to stay in Partequineus with them forever.
Vanessa is able to visit Partequineus whenever a purple mist invades her room at four o'clock. On one day-time visit she encounters five animated ponies. She rides one of them, but the dragon appears and tries to capture her. She escapes back to her room. On a night-time visit, she finds five boys in Partequineus. They explain to her that they are under a magic spell that transforms them into ponies by day, and back to boys at night. The girls become ponies at night.
Later, the girls tell Vanessa the history of Partequineus, about the first little girl who was captured and made to stay there. They also explain how the magic spell can be broken; one of them must reach the land of Paximus, which lies beyond a wide desert. But the dragon won't allow the children or the ponies to cross the desert.
Vanessa devises a plan whereby the girls will ride the boy ponies across the desert all at once, hoping that at least one of them can avoid the dragon. An exciting battle ensues. It seems as if the dragon will catch them all, and keep Vanessa in Partequineus forever. But Vanessa's plan works, and when the smallest child and the smallest pony reach the land of Paximus, the spell is broken.
Vanessa returns home, and comes to believe that it was all a dream. Then a new student arrives at her school, and they recognize each other from their time in Partequineus. It wasn't a dream.