After a wealthy philanthropist expresses an unusual interest in a ring found by her niece Annabelle, Miss Fritton explains that she's descended from a pirate who, in 1598, stole treasure from another: the philanthropist's ancestor. This discovery sends the racuous girls of St. Trinians on the hunt for buried treasure.

When their beloved school is threatened with closure should the powers that be fail to raise the proper funds, the girls scheme to steal a priceless painting and use the profits to pull St. Trinian's out of the red.