Alastair Sim holds sway over that most enduring of British institutions, the private girls school, triumphing in the dual role of the Headmistress, Miss Fritton, and her horseracing brother Clarence. Frank Launder and Sidney Gilliat were inspired by the hideous schoolgirls of Ronald Searle's popular comic strip of the early 1940s; the "belles" of the title are the unruly student body at a nearly bankrupt academy in the English countryside. Miss Fritton attempts to bail out the institution by investing its entire funds on Arab Boy, a race horse. Sim is supported in the usual sterling style by a cast featuring Joyce Grenfell as a jaunty police spy, George Cole as the chemist/brewer "Flash" Harry, and a host of very unlikely teachers and pupils.