Miss Fritton (Alastair Sim) is the headmistress of St. Trinian's school for girls. Her twin brother Clarence (Alastair Sim) is a little more shady, he is a bookmaker. When Princess Fatima comes to town to watch her father's horse Arab Boy run in the local Gold Cup, Clarence is making odds on the horse. Some of the school's students are betting against the horse and others for it, thus creating a conflict in their midst, while Miss Fritton is betting on the horse to try and save the school. Meanwhile Flash Harry (George Cole) capitalizes on the girls making alcohol in the school lab. On top of all this the police have infiltrated the school, in the form of teacher Ruby Gates (Jody Grenfell), due to the disappearance of some former agents. The whole thing is quite the mess to be sure.