Tamzin Weston unwittingly releases the evil spirit of the Grey Horse, who has been trapped inside a small statue for hundreds of years. Her mistake threatens to bring perpetual storms and danger. Only Tamzin can put things right and bring balance back to nature. To do this she must find and enlist the spirit of the Blue Horse, a force for good, who has battled and overcome the Grey Horse once before.

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.