Sandra and Adam are spending the summer in the country with gentle Miss Vaughan, a refuge after their angry uncle's crowded house in London. Adam is happy fishing, but Sandra envies the children at Greens, the riding holiday school, as she has only fat, lethargic Toby to ride. Meanwhile those children are disgusted with the new owners of Greens, the Moggs. Tired of being neglected and underfed, they decide to rebel, but have no idea they are riding into danger.