Summer is finally here! The Immenhof sisters Lou, Charly, and Emmie are looking forward to a relaxed time with their horses. There's just one catch – Immenhof is bankrupt. What's more, the girls are under constant attack from the mean owner of their neighboring stud farm and his arrogant horse trainer Runa. Runa is having trouble to control Cagliostro, the new star racehorse, and she accuses Lou of being responsible for the horse's difficult behavior. Lou only sees one way out: she agrees to a risky deal, in which she not only puts her favorite mare Holly on the line, but also risks the future of their entire stables.