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.

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.