When Tamzin goes to stay with her grandmother in Cornwall, she is drawn towards a small grey statue of a grey horse, kept in a cabinet for safe-keeping. One day she accidentally drops the statue and unwittingly releases the evil spirit of the Grey Horse that has been trapped inside. Her mistake threatens to bring storms and danger, and it is only Tamzin who can put things right. To do this she needs the help of the benevolent spirit of the Blue Horse who, according to the ancient legend, defeated the Grey Horse once before.
In this third adventure, a newcomer arrives and with his help Tamzin is nearer to solving the mystery behind the ancient legend. But the Grey Horse's power is growing even stronger, putting not only her life in danger but that of the whole village. Hoewever Tamzin is almost sure that the spirit of the Blue Horse is nearby when her favourite pony Moonlight comes to her rescue once again.