One night, an unknown man telephones St. Trinian's and bribes a girl, Celia, to find an old ring in the school library. She is caught by the other girls, and forced to explain that the man offered her Â£20,000 for the ring. Annabelle, the new head girl of St. Trinian's, bartered for Â£100,000 from the man when he called back, but he refused and then proceeded to threaten them. Afterwards, the girls ask the headmistress, Camilla Fritton, about why the ring might be so valuable. She tells them a story about Pirate Fritton, her ancestor, who supposedly hid a great treasure somewhere in the world. He had two rings made, which when fitted together would reveal the location of the treasure.
The man on the telephone who wanted the ring is Sir Piers Pomfrey, a member of a secret society, and descendant of a man robbed by Pirate Fritton in 1589. Sir Piers manages to steal the ring from headmistress Camilla. The girls decide to search for the second ring, and find a clue left by another ancestor of the Frittons, who found the first ring (left in the library) and believed he knew where to look for the second. After a difficult deciphering of the clue, a midnight hunt in a graveyard, dealing with demonic possession, and disguising themselves as boys to infiltrate another school, the girls find the elusive second ring at last.
Now, all that is left is to steal back the ring that Sir Piers Pomphrey took.
They begin by researching Sir Piers Pomphrey (who runs a secret society known as AD1 who are a masculinist brotherhood), who has a seemingly flawless reputation, and discover that he knows Geoffrey Thwaites â an old flame of Camilla Fritton from the previous film. Camilla tracks him down to ask for help, and he informs her that Sir Piers is the head of a secret society known only as AD1. Geoffrey is persuaded to join forces with the girls of St. Trinian's against Piers.
They organize an undercover attempt to break into the headquarters of AD1, recruiting former head girl Kelly (now an M.I.7 agent). They attempt to retrieve the first ring, only to discover that it is not in the secret society's vault. Luckily, Geoffrey realises where the ring really is, on Pomfrey's finger, and manages to get it back to St. Trinian's.
At last the two rings are together, and they reveal the treasure's location; Shakespeare's Globe. After organising a flash mob to keep AD1 from following them, a small group of the girls, their headmistress, and Geoffrey head for their new destination. They reach a secret room, and open a chest, only to find that there are no golden riches waiting for them. But what they do find is another treasure entirely: two surprising secrets held by Pirate Fritton. First; that he wrote plays under the name William Shakespeare, and second; that he was in fact a she, a secret the AD1 could never let be revealed.
Sir Piers Pomfrey reaches the girls and takes the evidence they have found, threatening them with a gun. Just as he is about to make a getaway down the Thames aboard his private boat, destroying the evidence, Celia (played by Juno Temple) has an idea. The girls sail the reconstruction of the Golden Hind down the Thames, while shooting cannons at Pomfrey's boat. Then, Miss Fritton swings down and swipes the evidence from Pomfrey's hands. The end shows St Trinian's throwing a wild party.
What was in the chest instead of riches?
Secrets