In the thirteenth book of the popular Heartland series, the horses come down with an equine flu and the entire stable is quarantined, forcing Amy to realize there is a limit to how much she can do.A fierce flu strain is infecting Heartland's horses, and, despite Amy and Ty's fervent efforts to treat and isolate those infected, the flu spreads to other horses and the vet quarantines Heartland. To make matters worse, the late summer heat heightens the tension. Ben is frustrated when Red starts to exhibit flu symptoms because their show season will be cut short. Ty thinks Ben is being selfish, and Amy feels caught in the middle. Then, just as some horses start to get better, a tornado devastates the valley. When the storm is over, everyone is forced to gain a new perspective.