Twelve-year-old Wendy is not at all happy that she has to spend her summer vacation with her parents at her grandma Herta's run-down riding stables. Wendy has been afraid of horses ever since a riding accident. But then she meets the pretty mare Dixie, who has run away from the butcher. Slowly, a deep friendship develops between the horse and the girl, and while Wendy does everything she can to save Dixie, she also manages to overcome her fear.

The three investigators want to go on a road trip during their vacation, but just as they are about to set off, the phone rings and an unknown caller hands the trio of detectives their latest case: the archaeologists belonging to the secret society Sphinx are carrying out illegal excavations and selling the stolen art treasures. Their next target is the supposedly cursed island of Makatao.

Her grandmother's riding stable, where Wendy lives with her parents, is on the verge of bankruptcy. When she hears about a youth tournament, she wants to take part so that she can use the prize money to save the farm. But she doesn't have the right horse. Her former circus horse Dixie is not ready to jump hurdles. The show horse Penny is traumatized. So Wendy has to look after Penny and make sure Dixie doesn't get jealous at the same time.