The Pony Detectives have a packed Christmas holiday ahead - they're riding in a show, doing a charity ride for Hope Farm and best of all, they're going to see their first-ever polo match at Fortnum Abbey with star Argentinian rider Estoni and his horse, Stormchaser. When the girls begin investigating the mystery of ponies being let loose from their fields, the clues lead them inside the closely guarded grounds of Fortnum Abbey. But as they close in on their suspect, events take an unexpected turn, leaving Stormchaser's life in the balance. The PDs have to act fast if they are to solve the case and save the star polo pony.