Carole is head girl at the Old Gatehouse School. She starts the Gatehouse Cavalry, who will perform a musical ride on the school ponies at shows and gymkhanas. Every rider in the school is keen to be chosen, particularly Gerry and her harebrained friend Jackie. The least keen of all is Angela, who detests horses and spends her time planning ways of escape from school. There is much adventure for these three and for Miss Tanner the hateful riding mistress, before the Cavalry rides, and Gerry becomes a heroine.