Annabelle Fritton (Talulah Riley) is horrified to discover that her new school is the infamous St. Trinian's, jam-packed with misfits and malcontents led by sultry Head Girl Kelly (Gemma Arterton). Annabelle has a difficult time adjusting until she overhears her father and the headmistress of the school (Rupert Everett in a dual role) discussing how she doesn't fit in with her own family. Once she realizes she's a misfit as well, Annabelle accepts her place at St. Trinian's. No sooner has she settled in than she realizes her aunt, the headmistress, is struggling to keep the school open due to financial difficulties and pressures from the education minister (Colin Firth). When the girls discover that the school is closing down, Kelly, Annabelle and the other St. Trinian's girls devise a daring plot to earn enough money to save their school.