With the students having saved St. Trinian's School for Young Ladies from bankruptcy and head girl Kelly Jones having moved onto some secret assignment, Miss Camilla Fritton, St. Trinian's headmistress, appoints her niece, Annabelle Fritton, as the school's new head girl, a role for which previously meek Annabelle isn't sure she is yet prepared to inherit. Annabelle's mettle in her new role is tested when fellow student Chelsea informs her and the rest of the students that she has been approached by an unknown man to retrieve a ring from the St. Trinian's long not touched archives for a payment of £20,000. The students learn from Miss Fritton that, if the four hundred twenty year old legend holds true, that ring is only one of two when placed together lead to the location of a gold treasure. That legend also includes an on the sea battle between a Captain Lord Pomfrey and a Captain Archibald Fritton, one of Camilla and Annabelle's ancestors. They will all learn that the identity of the buyer is Sir Piers Pomfrey, one of Lord Captain Pomfrey's descendants who must be after the treasure in large part to spite the Frittons. As such, the girls. with Annabelle leading the way, decide they have to beat Sir Pomfrey to the punch by protecting the ring they have on hand, finding the other half of the ring which requires figuring out the puzzle of the legend, and putting the two together to determine the location of the treasure and a way to retrieve it. Because of Sir Pomfrey's connections, the girls believe their task would be aided if Miss Fritton is willing to call in a favor, from now disgraced former Minster of Education and her former lover wannabe Geoffrey Thwaites. In the process, the girls also learn that Sir Pomfrey and the Pomfrey legacy has a grander overall picture, one that strikes to the very heart of the girls as people, and one that they should try to expose, which once again could prove challenging if only because of Sir Pomfrey's stellar reputation as opposed to theirs as delinquents of society.