David goes on a journey to the moon-garden where everything is beautiful and where he also meets Phyllis who is not like the other children in the garden. While he is allowed to play in this beautiful place for awhile, he at last finds out that he has been brought there to reveal his true mission, which is to find the Wonder-Box and the Know-All Book that is hidden in the Iron Castle and bring them back to earth. In order to find the Iron Castle, he must first find and tame the Black Winged Horse. Will he be able to succeed at the task given him?