The ten-year twins are delighted when a scruffy brown pony appears almost on their doorstep in the summer holiday. Reuben, a bit of a dare-devil, or stupid as his sister Esme says, decides to ride Mr Nibbs.
That was it! The twin's lives change forever. It turns out that Mr Nibbs is a special agent sent to recruit them into a world of different realities.
Old Woman, who is also Young Man, and an Earth Guardian, has noted the children have the qualities needed to join the elite corps of young spiritual warriors training to protect the Earth from further human harm. The mythical flying horse Pegasus is called in to assist with their training.
Through their adventures with Pegasus they meet all sorts of beings. Eventually some friends are brought in for support, including the girl, Rose, Esme's strange friend. Esme's dream animals walk with her in the dense three dimensional world of ordinary human reality.
There is one big problem. Once recruited, the children come to the attention of the Magnificent Master of Malicious Magic. This unhappy Being feeds off fear and wants the world to remain full of chaos. He doesn't want the children to succeed. They will need protecting!