Celia (Juno Temple) is offered a nice price for a ring she found in an old school library by millionaire Piers Pomfrey (David Tennant). The ring and ancient heirloom hides a secret to a treasure buried by pirates. A search for the treasure is launch by the headmistress of St Trinians, Miss Fritton. The girls of St. Trinians school are all involved in the intense search for the pirates booty after having discovered that Fortnam Frittin (Rupert Everett) is related to a famous pirate. The evil Piers is there to stop them and recover the treasure for himself, but the girls are persistent and turn out to be formidable opponents in the quest for the treasure.