First in the series of films based on the cartoon creations of Ronald Searle. Miss Millicent Fritton (Alastair Sim), headmistress of St Trinian's School for Girls, attempts to stave off her creditors by 'looking after' the pocket money of a wealthy sheikh's daughter currently enrolled at the school, and investing it on the sheikh's horse, Arab Boy, in the local derby. Unfortunately, Miss Fritton's bookmaking brother Clarence (also Sim) has backed another horse, and kidnaps Arab Boy to ensure his own sizeable win. Miss Fritton enlists the help of disreputable boot boy Flash Harry (George Cole), the teaching staff and the Old Girls in rescuing Arab Boy and returning him to the race.